a study on the production and properties of in-situ...
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A STUDY ON THE PRODUCTION AND PROPERTIES OF IN-SITU TITANIUM DIBORIDE PARTICULATE REINFORCED
ALUMINUM A356 ALLOY COMPOSITE
A THESIS SUBMITED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCE
OF MIDDLE EAST TECHNICAL UNIVERSITY
BY
OSMAN SERDARLI
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF MASTER OF SCIENCE IN
METALLURGICAL AND MATERIALS ENGINEERING
JUNE 2011
ii
Approval of the thesis:
A STUDY ON THE PRODUCTION AND PROPERTIES OF IN-SITU TITANIUM DIBORIDE PARTICULATE REINFORCED ALUMINUM A356 ALLOY
COMPOSITE submitted by OSMAN SERDARLI in partial fulfillment of the requirements for the degree of Master of Science in Metallurgical and Materials Engineering, Middle East Technical University by Prof. Dr. Canan Özgen Dean, Graduate School of Natural and Applied Sciences
Prof. Dr. Tayfur Öztürk Head of Department, Metallurgical and Materials Engineering
Prof. Dr. Naci Sevinç Supervisor, Metallurgical and Materials Engineering Dept., METU
Prof. Dr. Ali Kalkanlı Co-Supervisor, Metallurgical and Materials Engineering Dept., METU
Examining Committee Members: Prof. Dr. Hakan Gür Metallurgical and Materials Engineering Dept., METU
Prof. Dr. Naci Sevinç Metallurgical and Materials Engineering Dept., METU
Prof. Dr. Ali Kalkanlı Metallurgical and Materials Engineering Dept., METU
Prof. Dr. Erdoğan Tekin Metallurgical and Materials Engineering Dept., ATILIM UNI.
Assoc. Prof. Dr. Arcan Dericioğlu Metallurgical and Materials Engineering Dept., METU
Date:
30.06.2011
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I hereby declare that all information in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work.
Name, Last name: OSMAN SERDARLI
Signature:
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ABSTRACT
A STUDY ON THE PRODUCTION AND PROPERTIES OF
IN-SITU TITANIUM DIBORIDE PARTICULATE REINFORCED
ALUMINUM A356 ALLOY COMPOSITE
Serdarlı, Osman
M.Sc., Department of Metallurgical and Materials Engineering
Supervisor: Prof. Dr. Naci Sevinç
Co-Supervisor: Prof. Dr. Ali Kalkanlı
June 2011, 97 Pages
TiB2 particle reinforced aluminum matrix composites have been the subject of
several investigations. An M.Sc. thesis on production of TiB2 reinforced aluminum
composites by reaction between liquid aluminum and B2O3 and TiO2 dissolved in
cryolite has been completed in this Department in 2005. This study is a
continuation of the mentioned M.Sc.study. Composition of the starting cryolite-
B2O3-TiO2 system, temperature and time were used as experimental variables. The
resulting composite was squeeze cast and its microstructure was examined.
Mechanical properties of the produced composite were measured and how
mechanical properties of the composite vary with TiB2 content of the composite
was determined.
Keywords: In-Situ TiB2, Metal Matrix Composite, Particles Reinforcement
Composite
v
ÖZ
TİTANYUM DİBORÜR PARÇACIK TAKVİYELİ A356 ALÜMİNYUM
ALAŞIM KOMPOZİTİNİN ÜRETİM ÇALIŞMASI
Serdarlı, Osman
Yüksek Lisans., Metalurji ve Malzeme Mühendisliği Bölümü
Tez Yöneticisi: Prof.Dr.Naci Sevinç
Ortak Tez Yöneticisi: Prof.Dr.Ali Kalkanlı
Haziran 2011, 97 Sayfa
TiB2 parçacık takviyeli aluminyum matrisli kompozitler çeşitli çalışmalara konu
olmuştur. Bölümümüzde 2005 yılında biten bir M.Sc. çalışmasında sıvı
aluminyum ile kriyolit içinde çözünmüş B2O3 ve TiO2 nin reaksiyonu sonucu TiB2
parçacık takviyeli aluminyum kompozitlerin üretimi çalışılmıştır. Bu çalışma
bahsedilen M.Sc. çalışmasının devamıdır. Kriyolit-B2O3-TiO2 sisteminin başlangıç
bileşimi, sıcaklık ve süre deneysel değişkenler olarak kullanılmıştır. Elde edilen
TiB2 takviyeli aluminyum kompozit ezme döküm yöntemi ile dökülmüş ve
kompozitin içyapısı incelenmiştir. Elde edilen kompozitin mekanik özellikleri
ölçülmüş ve mekanik özelliklerin kompozitin içerdiği TiB2 miktarı ile değişimi
belirlenmiştir.
Anahtar Kelimeler: Yerinde Üretilmiş TiB2, Metal Matriksli Kompozit, Partikül
Takviyeli Kompozit
vi
T o my pare nts
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AKNOWLEDGMENTS
I wish to express here my deep and sincere gratitude to my professors: Dr.Ali
KALKANLI and Dr.Naci SEVİNÇ for their valuable guidance and encouraging
supports during this research.
Also offer my especial thank to Prof. Dr. Erdoğan TEKİN for her pure, honest,
benefited hints, without which I may not have been able to complete my master.
Mr. Salih TÜRE and all respectful staff of department’s laboratory are gratefully
acknowledged for their friendly helps.
My sincere and special gratitude for my parents who supported enlightened me
through the whole long way of my study.
Finally, I would never be able to thanks my real true friends Barış AKGÜN and
Pelin MARADİT their help in editing my research.
My appreciation to all of my friends who gave me hope and courage.
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TABLE OF CONTENTS
ABSTRACT ....................................................................................................... iv
ÖZ ....................................................................................................................... v
AKNOWLEDGMENTS .................................................................................... vii
TABLE OF CONTENTS .................................................................................. viii
LIST OF TABLES .............................................................................................. xi
LIST OF FIGURES ........................................................................................... xii
CHAPTERS
1.INTRODUCTION ............................................................................................ 1
2.LITERATURE SURVEY ................................................................................. 3
2.1 Introduction........................................................................................... 3
2.2 Metal Matrix Composite........................................................................ 4
2.3 Materials Selection ................................................................................ 7
2.3.1 Matrix Selection ................................................................................. 7
2.3.1.1 Aluminum Casting Alloys ............................................................... 9
2.3.1.2 Al-Si Casting alloys ....................................................................... 10
2.3.2 Reinforcement Selection................................................................... 12
2.4 Physical and Mechanical Properties of Metal Matrix Composites ........ 15
ix
2.4.1 Physical Properties ........................................................................... 15
2.4.2 Mechanical Properties ...................................................................... 16
2.4.2.1 Strengthening by Particles ............................................................. 16
2.4.2.2 Young’s Modulus (Modulus of Elasticity) ..................................... 18
2.5 Thermodynamics of Aluminum Matrix Composites ............................ 19
2.6 Solubilities of Phases in Cryolite and Liquid Aluminum...................... 20
2.6.1 Al2O3–Na3AlF6 and TiO2–Na3AlF6 binary systems ........................... 20
2.6.2 TiB2―Al system .............................................................................. 21
2.6.3. Al―Ti, Al―B, Ti―B binary systems and Al―Ti―B ternary system
................................................................................................................. 22
2.7 Processing Methods for Particulate Reinforced Al Alloy Matrix MMCS
................................................................................................................. 25
2.7.1 Liquid State Processes ...................................................................... 27
2.7.1.1 Casting .......................................................................................... 27
2.7.1.2 Pressure Infiltration Casting .......................................................... 28
2.7.1.3 In-Situ ........................................................................................... 29
2.7.1.4 Wettability between Reinforcement and Matrix ............................. 30
2.8 Solidification of Aluminum Matrix Composites and Distribution of
Ceramic Particles ...................................................................................... 32
2.9 Interfaces in Aluminum Matrix Composites ........................................ 34
3.EXPERIMENTAL PROCEDURE .................................................................. 35
3.1 Introduction......................................................................................... 35
x
3.2 Starting Materials ................................................................................ 36
3.3 Production Parameters ......................................................................... 36
3.4 Casting Practice and Squeeze Casting Machine ................................... 38
3.5 Determination of Parent Phases in A356-TiB2 Composite via X-Ray
Diffraction Analysis .................................................................................. 41
3.6 Metallographic Examination ............................................................... 41
3.7 Secondary Dendrite Arm Spacing Distance (SDAS) Measurement ...... 41
3.8 Mechanical Tests................................................................................. 43
3.8.1 Tensile Test ................................................................................... 43
3.8.2 Three Point Bending Test............................................................... 44
3.8.3 Vickers Micro-Hardness Test ......................................................... 45
4.RESULTS AND DISCUSSION...................................................................... 46
4.1 Formation of In-Situ TiB2 Reinforcing Particles and X-Ray Studies .... 46
4.2 Determination the Amount of TiB2 Reinforcement .............................. 57
4.3 Microstructures and Phase Relations ................................................... 60
4.4 Secondary Dendrite Arm Spacing (SDAS) Measurement .................... 77
4.5 Mechanical Properties of A356 –TiB2 Composites .............................. 81
5.CONCLUSIONS ............................................................................................ 90
REFERENCES .................................................................................................. 92
xi
LIST OF TABLES
TABLES
Table 2.1. The Composition of Common Al-Si Alloys * [16] .............................. 11
Table 2.2. The Chemical Composition of 356.0 Al-Si casting alloy ..................... 12
Table 2. 3. Essential Properties of Selected Reinforcements [21],[11],[22]. ......... 13
Table 2.4 Surface and interface strains of selected metal–ceramic systems at
different temperatures ......................................................................................... 31
Table 3. 1 Pre-heating Die Temperatures ............................................................ 40
Table 4.1 Diffractometric Data for Pure Al-TiB2 As-Cast Composite ................. 48
Table 4.2 Diffractometric Data for A356-TiB2 Squeeze Cast Composite ............. 54
Table 4.3 Diffractometric Data of TiB2 Leach Residue ........................................ 58
Table 4.4 The Efficiencies of the TiB2 production via Slag-Metal reaction .......... 59
Table 4.5. Chemical Compositions of Prepared Metal Matrix Composite Samples
........................................................................................................................... 60
Table 4.6 Reinforcement Content and SDAS Size* ............................................. 81
Table 4.7 Mechanical Testing Results of A356-TiB2 Composite ......................... 81
Table 4. 8 Brinell Macro and Vickers Micro Hardness Numbers of A356-TiB2
Composite ........................................................................................................... 82
Table 4.9 Comparison of Mechanical Properties of Al-TiB2 Metal Matrix
Composites ......................................................................................................... 89
xii
LIST OF FIGURES
FIGURES
Figure 2. 1 Common forms of fiber reinforcement. In general, the reinforcements
can be straight continuous fibers, discontinuous or chopped fibers, particles or
flakes, or continuous fibers that are woven, braided, or knitted [6]. ....................... 4
Figure 2. 2 Classification of the composite materials within the group of materials
[8]. ........................................................................................................................ 6
Figure 2.3 Aluminum-silicon phase diagram and cast microstructures of pure
components and of alloys of various compositions. Alloys with less than 11.6% Si
are referred to as hypoeutectic, those with close to 11.6% Si as eutectic, and those
with over 11.6% Si as hypereutectic. ................................................................... 11
Figure 2. 4 Alumina Solubility in Cryolite [42] ................................................... 20
Figure 2. 5 Cryolite-Titanium dioxide System [43].............................................. 21
Figure 2. 6 Al-Ti binary phase diagram [36]. ....................................................... 22
Figure 2. 7 Al-B binary phase diagram [45]......................................................... 23
Figure 2. 8 Ti-B binary phase diagram [45]. ........................................................ 24
Figure 2. 9 Al-B-Ti Ternary phase diagram [45].................................................. 25
Figure 2. 10 Primary processing techniques of Al-MMCs according to physical
state of matrix ..................................................................................................... 26
Figure 2. 11 Schematic of structure of the top-fill PIC method ............................ 28
Figure 2. 12 Edge angle adjustment of a melt drop on a solid base for various
values of the interface energy (after Young) [50]. ................................................ 30
xiii
Figure 3. 1 The schematic illustration of the slag-metal system in the graphite
crucible .............................................................................................................. 37
Figure 3. 2 The hot worked tool steel die is designed two tensile specimen and
four bending specimen ........................................................................................ 39
Figure 3. 3 Squeeze casting machine with hot worked steel die ........................... 39
Figure 3. 4 (200) crystal plane peak of Aluminum ............................................... 43
Figure 3.5 The schematic of the tensile test specimen produced by
squeeze/pressure die casting machine .................................................................. 43
Figure 3. 6 The schematic of the three point bending test specimen specimens
produced by squeeze/pressure die casting machine .............................................. 45
Figure 4. 1 Al- 0% vol. TiB2 Commercially Pure Aluminum ............................. 49
Figure 4. 2 Al- 5% vol. TiB2 and 25 min. reaction time ....................................... 49
Figure 4. 3 Al- 5% vol. TiB2 and 45 min. reaction time ....................................... 50
Figure 4. 4 Al- 5% vol. TiB2 and 70 min. reaction time ....................................... 50
Figure 4. 5 Al- 7.5% vol. TiB2 and 25 min. reaction time .................................... 51
Figure 4. 6 Al- 7.5% vol. TiB2 and 45 min. reaction time .................................... 51
Figure 4. 7 Al- 7.5% vol. TiB2 and 70 min. reaction time .................................... 52
Figure 4. 8 Al- 10% vol. TiB2 and 25 min. reaction time ..................................... 52
Figure 4. 9 Al- 10% vol. TiB2 and 45 min. reaction time ..................................... 53
Figure 4. 10 Al- 10% vol. TiB2 and 70 min. reaction time ................................... 53
Figure 4. 11 A356 -0% vol.TiB2 .......................................................................... 55
Figure 4. 12 A356 -5% vol.TiB2 .......................................................................... 55
xiv
Figure 4. 13 A356 -7.5% vol.TiB2 ....................................................................... 56
Figure 4. 14 A356 -10% vol.TiB2 ........................................................................ 56
Figure 4. 15 XRD pattern of leach residue of TiB2 powder .................................. 58
Figure 4. 16 XRD pattern of slag phase containing Al2O3 and Na3AlF6 ............... 59
Figure 4. 17 Optical micrographs of A356 alloy with different magnifications
respectively; (a) X100, (b) X200, (c) X500, and (d) X1000, Samples were etched
with Keller’s reagent ........................................................................................... 61
Figure 4. 18 Optical micrographs of A356-5vol. % TiB2 with different
magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000,
Samples were etched with Keller’s reagent.......................................................... 62
Figure 4. 19 Optical micrographs of A356-7.5 vol. % TiB2 with different
magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000 ,
Samples were etched with Keller’s reagent.......................................................... 63
Figure 4. 20 Optical micrographs of A356-10 vol. % TiB2 with different
magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000 ,
Samples were etched with Keller’s reagent.......................................................... 64
Figure 4. 21 Optical micrographs of annealed (at 450°C for 20 hours),A356
reference specimen with different magnifications respectively; (a) X100, (b) X200,
(c) X500, and (d) X1000 , Samples were etched with Keller’s reagent ................ 65
Figure 4. 22 A356 0% vol TiB2 Squeeze cast, Samples were etched with Keller’s
reagent ................................................................................................................ 68
Figure 4. 23 A356-5% vol TiB2 , Squeeze cast Samples were etched with Keller’s
reagent ................................................................................................................ 69
Figure 4. 24 A356-7.5% vol TiB2 , Squeeze cast, Samples were etched with
Keller’s reagent ................................................................................................... 70
xv
Figure 4. 25 A356-10% vol TiB2 , Squeeze cast Samples were etched with Keller’s
reagent ................................................................................................................ 71
Figure 4. 26 EDS point analyses on Figure 4.22a ................................................ 73
Figure 4. 27 EDS point analyses on vol.5% TiB2-A356 Alloy at Figure 4.23a ..... 75
Figure 4. 28 EDS point analyses on vol. 7.5% TiB2-A356 Alloy at Figure 4.24b . 76
Figure 4. 29 EDS point analyses on vol. 10% TiB2-A356 Alloy at Figure 4.25b . 77
Figure 4. 30 Diffraction patterns of A356-TiB2 composite showing (200) peaks
broadening because of secondary dendrite arm spacing’s .................................... 78
Figure 4. 31 Dendrite arm spacing measurement of A356 reference sample via to
special metallographic light microscope and software. X200 magnification and
etched with Keller’ reagent ................................................................................. 80
Figure 4. 32 (a) Variation of ultimate tensile strength (UTS) with volume percent
of reinforcements.(b)Variation of yield strength (YS) with volume percent of
reinforcements.(c)Variation of flexural strength with volume percent of
reinforcements.(d)Variation of hardness (HV0.2) with volume percent of
reinforcements. (e) Variation of elastic modulus (E) with volume percent of
reinforcements .................................................................................................... 83
Figure 4. 33 Fracture surfaces of the defected tensile specimens.......................... 86
Figure 4. 34 Hydrogen solubility (pH2=1 atm) with increasing temperature [64] . 86
Figure 4. 35. (a)Variation of elongation with volume percent of reinforcements.
(b)Variation of ductility with volume percent of reinforcements .......................... 87
1
CHAPTER I
INTRODUCTION
In recent years, popularity of composite materials has drastically increased, against
to the conventional monolithic alloys. Today, efficient materials, in terms of high
specific strength and elastic modulus with light weight, are desired by aerospace,
automotive and military applications. Savings in weight result in savings in fuel
which translates directly into substantial increase in payload capabilities. A 10%
saving in aircraft structural weight increases the available reserve payload by 4%
[1]. Metal Matrix Composites, are composed of metallic matrix which provides
ductility and toughness and generally ceramic reinforcements which provide high
strength and high modulus. They offer a wide range of attractive material
properties, both mechanical and physical, that cannot be achieved using
conventional engineering alloys. These enhanced materials properties are the
direct result of the interaction between the metallic matrix and the reinforcement
[2].
Early studies on Metal Matrix Composites (MMCs) have especially concentrated
on continuous fiber or short fiber reinforced MMCs. Even though, they have
excellent mechanical and physical properties, production cost, complex production
routes, reinforcement-particle interface integrity and anisotropic physical and
mechanical properties cause some limitations. On the other hand, discontinuous
particulate reinforced MMCs achieve some restrictions that continuous reinforced
MMCs cannot succeed. Discontinuously reinforced MMCs can be manufactured
either in solid state (powder metallurgy.) or in liquid state (stir casting, in-situ)
processes and they can be subjected to secondary forming operations (extrusion,
2
rolling, forging). Furthermore, they exhibit isotropic mechanical properties
(especially UTS, Yield S.) . More recently, this class of metal matrix composites
has attracted considerable attention as result of: (a) availability of various type of
reinforcements at competitive costs, (b) the successful development of
manufacturing processes to produce metal matrix composites with reproducible
structures and properties, and (c) the availability of standard or near standard metal
working methods which can be utilized to form theses metal matrix composite [3].
In this study, in-situ TiB2 particulate reinforced Aluminum alloy matrix
composites were produced via reaction of liquid Al with a slag produced from
TiO2, H3BO3 and Na3AlF6. Composition of the starting slag and reaction time were
used as a experimental variables and TiB2 reinforced aluminum composites having
different volume percentages of TiB2 were produced. Microstructural and
mechanical properties of the produced composites were determined and their
relationship with the TiB2 amount and distribution was discussed.
Existing and potential Applications of Al-alloy/TiB2 Particulate Reinforced
Composites
(a) Prototype Parts and Alloys:
(a1) Automobile conrod: Wrought Alloy 2618 + TiB2 Fatigue tests on forged
demonstrator parts
(b) Aerospace & Automotive Applications
(b1) Helicopter Blade Sleeve Application: Wrought Alloy 2618 + TiB2 Tests on
pancake samples and forged I-beam
(b2) Seat Track Application: Wrought Alloy 7075 + TiB2 Continuous cast and
extruded; heat-treated
(c) Wear Resistance Applications: Casting Alloy A356 + TiB2 Cylinder liners
for automotive engines produced by high-pressure die casting and tested under
realistic conditions in co-operation with Al producer and automotive companies.
3
CHAPTER II
LITERATURE SURVEY
2.1 Introduction
Metal Matrix Composites (MMCs), which are new and important class of
materials for aerospace, automotive and structural wear applications, have become
popular in the last decades. They offer a wide range of attractive material
properties, both mechanical and physical, that cannot be achieved using
conventional engineering alloys. These enhanced materials properties are the
direct result of the interaction between the metallic matrix and the reinforcement
[4].
In general, three common synthesizing routes are available for Metal Matrix
Composites; Solid-State process, Liquid-State process and Two-Phase process.
Among these, the liquid-state process yields better physically and chemically
stable matrix-reinforcement interface and fine microstructural homogeneity. These
two terms directly affect ambient and high temperature mechanical properties of
metal matrix composite. Therefore, the clear interface is directly related with
mechanical properties. In addition to these advantages, fast, low cost, single step
composite synthesizing is achieved by the liquid-state process.
In-Situ Process is a novel technique that does not require any secondary process
parameters. In this process the required reinforcing phases are produced by
chemical reaction among the appropriate ingredients at suitable temperatures.
During the last decade new in-situ fabrication technologies for processing metal
and ceramic composites have been developed by chemical reactions generating
4
very fine, thermodynamically stable reinforcing ceramic phases [5]. In-situ process
also results in uniform distribution of the reinforcement and clean interface
between matrix and reinforcement.
2.2 Metal Matrix Composite
Composites are commonly classified at two distinct levels. The first level of
classification is usually made with respect to the matrix constituent. The major
composite classes include organic-matrix composites (OMCs), metal-matrix
Composites (MMCs), and ceramic-matrix composites (CMCs). The term “organic-
matrix composite” is generally assumed to include two classes of composites:
polymer-matrix composites (PMCs) and carbon-matrix composites (commonly
referred to as carbon-carbon composites).
The second level of classification refers to the reinforcement form: particulate
reinforcements, whisker reinforcements, continuous fiber laminated composites,
and woven composites (braided and knitted fiber architectures are included in this
category), as depicted in Figure 2.1 [6].
Figure 2. 1 Common forms of fiber reinforcement. In general, the reinforcements can be straight continuous fibers, discontinuous or chopped fibers, particles or flakes, or continuous fibers that are woven, braided, or knitted [6].
5
Reinforcement is considered to be a “particle” if all of its dimensions are roughly
equal. Thus, particulate-reinforced composites include those reinforced by spheres,
rods, flakes, and many other shapes of roughly equal axes. Whisker
reinforcements, with an aspect ratio typically between approximately 20 to 100,
are often considered together with particulates in MMCs. Together, these are
classified as “discontinuous” reinforcements, because the reinforcing phase is
discontinuous for the lower volume fractions typically used in MMCs [6].
“Reinforcement of metals can have many different objectives. The reinforcement
of light metals opens up the possibility of application of these materials in areas
where weight reduction has first priority. The precondition here is the
improvement of the component properties. The development objectives for light
metal composite materials are” [7]:
Increase in yield strength and tensile strength at room temperature and
above while maintaining the minimum ductility and toughness,
Increase in creep resistance at higher temperatures compared to that of
conventional alloys,
Increase in fatigue strength, especially at higher temperatures,
Improvement of thermal shock resistance,
Improvement of corrosion resistance,
Increase in Young’s modulus,
Reduction of thermal elongation.
6
Important physical and chemical properties of three main materials are shown in
Figure 2.2. Metal matrix composite materials are intersections of these properties
Figure 2. 2 Classification of the composite materials within the group of materials [8].
Discontinuous term usually refers to particulate reinforced composites because the
ceramic phase discontinuously reinforced metal matrix. In a DRMMC
(Discontinuously Reinforced Metal Matrix Composite) materials system, the
reinforcement strengthens the metal matrix both extrinsically, by load transfer to
the ceramic reinforcement, and intrinsically, by increasing dislocation density [9].
The interaction between the particulate reinforcement and the metallic matrix is
Metallic Materials
Predominantly:
Metallic bond Crystalline structure Good conductivity Ductile Chemically unstable
Ceramic Materials
Predominantly:
Crystalline Structure Poor conductivity Chemically Stable
COMPOSITES
Non Metallic Materials
Predominantly:
Homopolar and dipolar bond amorphous structure
Poor conductivity High temperature
ductile Chemically stable
7
the basis for the enhanced physical and materials properties associated with
DRMMC materials systems. Composite materials properties can be tailored to
meet specific engineering requirements by selecting a particular reinforcement and
varying the amount added to the metal matrix. In this fashion, the physical and
mechanical properties of the composite materials system can be controlled with
some independence.
Increasing the reinforcement volume in a composite system increases mechanical
properties, such as elastic modulus, ultimate strength, and yield strength, while
reducing the thermal expansion and, in some cases, the density of the composite
system. Unfortunately materials properties such as ductility and fracture toughness
typically decrease with increasing reinforcement volume. The increase in both the
elastic modulus and strength (ultimate and yield) is believed to be due to the
difference in thermal expansion between the ceramic reinforcement particles and
the metallic matrix during processing. During the production of these composites,
both the reinforcement and matrix are heated to processing temperature, brought to
thermomechanical equilibrium, and then allowed to cool. The thermal contraction
of the metallic matrix during cool down is typically much greater than that of the
reinforcement, which leads to a geometric mismatch. At the ceramic-metal
interface, this geometrical disparity creates mismatch strains that are relieved by
the generation of dislocations in the matrix originating from sharp features on the
ceramic reinforcement [10].
2.3 Materials Selection
2.3.1 Matrix Selection
Wide range of metals can be used for the matrix of the composites whereas
lightweight and compatibility with the ceramic reinforcements restricts choice of
matrix metal. The common material candidates are Aluminum, Magnesium,
Titanium, Copper, Zinc, Nickel and Lithium. Another restriction is that chemical
reactions should not take place between matrix and reinforcement. Such a reaction
deteriorates reinforcement-matrix interface properties hence the mechanical
8
properties are reduced. Aluminum is the best option to produce in-situ TiB2
reinforcement composite due to reasonable mechanical, casting properties and
good compatibility of TiB2 reinforcement.
The requirements of low density, with reasonably high thermal conductivity, have
made aluminum and magnesium alloys the most commonly used matrices.
Regarding alloying additions, the result of several studies have shown that low
matrix alloying additions result in metal matrix composites with attractive
combinations of strength, ductility and toughness. Minor alloying elements
commonly used in wrought alloys as grain refiners, are unnecessary in
discontinuous reinforced metal matrix composites. Furthermore, these additions
should be avoided, since they may result in the formation of coarse intermetallic
compounds during consolidation and subsequence processing, thus impairing the
ductility of the composite [11],[12].
It is convenient to divide aluminum alloys into two major categories: casting
compositions and wrought compositions. Cast and wrought alloy nomenclatures
have been developed. The Aluminum Association system is most widely
recognized in the United States. Their alloy identification system employs
different nomenclatures for wrought and cast alloys, but divides alloys into
families for simplification [13].
For wrought alloys a four-digit system is used to produce a list of wrought
composition families as follows:
1xxx Controlled unalloyed (pure) compositions,
2xxx Alloys in which copper is the principal alloying element, though
other elements, notably magnesium, may be specified,
3xxx Alloys in which manganese is the principal alloying element,
4xxx Alloys in which silicon is the principal alloying element,
5xxx Alloys in which magnesium is the principal alloying element,
6xxx Alloys in which magnesium and silicon are principal alloying
elements,
9
7xxx Alloys in which zinc is the principal alloying element, but other
elements such as copper, magnesium, chromium, and zirconium may
be specified,
8xxx Alloys including tin and some lithium compositions
characterizing miscellaneous compositions,
9xxx Reserved for future use.
2.3.1.1 Aluminum Casting Alloys
Aluminum casting alloys are the most versatile of all common foundry alloys and
generally have the highest castability ratings. As casting materials, aluminum
alloys have the following favorable characteristics [14]:
Good fluidity for filling thin sections,
Low melting point relative to those required for many other metals,
Rapid heat transfer from the molten aluminum to the mold, providing
shorter casting cycles,
Hydrogen is the only gas with appreciable solubility in aluminum and
its alloys, and hydrogen solubility in aluminum can be readily
controlled by processing methods,
Many aluminum alloys are relatively free from hot-short cracking and
tearing tendencies,
Chemical stability,
Good as-cast surface finish with lustrous surfaces and little or no
blemishes.
Casting compositions are described by a three-digit system followed by a decimal
value. The decimal .0 in all cases pertains to casting alloy limits. Decimals .1 and
.2 concern ingot compositions, which after melting and processing should result in
chemistries conforming to casting specification requirements. Alloy families for
casting compositions are:
1xx.x Controlled unalloyed (pure) compositions, especially for rotor
manufacture,
10
2xx.x Alloys in which copper is the principal alloying element, but other
alloying elements may be specified,
3xx.x Alloys in which silicon is the principal alloying element, but other
alloying elements such as copper and magnesium are specified,
4xx.x Alloys in which silicon is the principal alloying element,
5xx.x Alloys in which magnesium is the principal alloying element,
6xx.x Unused,
7xx.x Alloys in which zinc is the principal alloying element, but other
alloying elements such as copper and magnesium may be specified,
8xx.x Alloys in which tin is the principal alloying element,
9xx.x Unused.
2.3.1.2 Al-Si Casting alloys
Some 238 compositions for foundry aluminum alloys have been registered with
the Aluminum Association. Although only 46% of this total consists of aluminum-
silicon alloys, this class provides nearly 90% of all the shaped castings
manufactured. The reason for the wide acceptance of the 3xx.x alloys can be found
in the attractive combination of physical properties and generally excellent
castability. Mechanical properties, corrosion resistance, machinability, hot tearing
resistance, fluidity, and weldability are considered the most important [15]. This is
why Al-Si casting alloy is selected as a matrix alloy in the presnt study. The
compositions of most common casting alloys are summarized in Table 2.1 While
these alloys exhibit excellent wear resistance, pressure tightness, fluidity, and
shrinkage, their machinability and weldability characteristics are not as good [16].
11
Table 2.1. The Composition of Common Al-Si Alloys * [16]
Alloy Fabrication
Method ǂ
Element (wt.%)
Si Cu Mg Fe Zn Others 350
Series S/PM 5-7 <0.2-1.25 0.35-0.55
<0.06-
<0.2
<0.1-
<0.35 ----
380
Series D
8.5-
11.0 2.0-3.5 <0.1-<0.3 <1.3 <3.0 <0.3Sn
390
Series D <7.5 4.5 0.55 <1.3 <0.1 <0.1Mg
400
Series S/PM/D
5.25-
12.0 <0.1-<0.3 <0.5-<0.1 <0.8-<2.0 <0.5 ----
* The remainder of the composition is aluminum and impurities. ǂ S- Sand casting; PM-Permanent mold casting; D-Pressure die casting
Aluminum-Silicon phase diagram is shown below. (Figure 2.3) Alloy
compositions and phase identifications are made according to this diagram.
Figure 2.3 Aluminum-silicon phase diagram and cast microstructures of pure components and of alloys of various compositions. Alloys with less than 11.6% Si are referred to as hypoeutectic, those with close to 11.6% Si as eutectic, and those with over 11.6% Si as hypereutectic.
12
Selection of suitable aluminum cast matrix alloy is 356.0. Chemical compositions
is shown below at the Table 2.2
Table 2.2. The Chemical Composition of 356.0 Al-Si casting alloy
Element (wt.%)
Si Mg Cu Mn Zn Fe Ti Al
6.5- 7.5 0.25-0.45 < 0.20 <0.10 < 0.10 <0.20 < 0.20 Balance
Alloy 356.0 (7 Si, 0.3 Mg) has excellent casting characteristics and resistance to
corrosion. This justifies its use in large quantities for sand and permanent mold
castings. Several heat treatments are used and provide the various combinations of
tensile and physical properties that make it attractive for many applications. This
includes many parts in both the automotive and aerospace industries. The
companion alloy of A356.0 with lower iron content affords higher tensile
properties in the premium-quality sand and permanent mold castings. [17]. Casting
characteristics, specific strength and wide commercial practice are main reasons
for selection of A356 alloy as a matrix.
2.3.2 Reinforcement Selection
The primary duty of reinforcement particles in metal matrix composites (MMCs)
is resisting and transferring the external load acting on the composite at the
boundary of particle and matrix [18]. Suitable wetting is a fundamental condition
for the combination of reinforcement particles with the aluminum matrix
throughout the composite casting. This favors the transfer and distribution of
stresses in the solid state without fracture of the composite. These combinations
could be formed by dissolution or chemical reaction of reinforcement particles
with matrix metal. However, chemical reactions are usually harmful to the
mechanical properties [19].
13
Selection criteria for the ceramic reinforcement include:
elastic modulus, tensile strength, hardness density, melting temperature, thermal stability, coefficient of thermal expansion, size and shape, compatibility with matrix material, and cost.
Some selected properties of commonly used ceramic reinforcements are shown in
Table 2.3. The structural efficiency of discontinuously reinforced metal matrix
composites is a function of the density, elastic modulus, and tensile strength of the
reinforcing phases [20] .
Table 2. 3. Essential Properties of Selected Reinforcements [21],[11],[22].
Reinforcement Density (g/cm3)
CTE (μm/m K)
Melting point (°C)
Strength (MPa)
Modulus (Gpa)
Al2O3 3.98 7.92 2015 221(1090°C) 379(1090°C)
AlN 3.26 4.84 2069 2069(24°C) 310(1090°C)
BeO 3.01 7.38 2570 24(1090°C) 190(1090°C)
BN 3.48 7.50 2327 2500 195
B4C 2.52 6.08 2450 2759(24°C) 448(24°C)
C 2.18 -1.44 3527 - 690
CeO2 7.13 12.42 2600 589(24°C) 185(24°C)
HfC 12.2 6.66 3928 - 317(24°C)
MgO 3.58 11.61 2620 41(1090°C) 317(1090°C)
MoSi2 6.31 8.91 2030 276(1090°C) 276(1260°C)
Mo2C 8.90 5.81 2500 - 228(24°C)
NbC 7.60 6.84 3500 - 338(24°C)
Si 2.33 3.06 1414 - 112
SiC 3.21 5.40 2830 - 324(1090°C)
Si3N4 3.18 1.44 1900 - 207
SiO2 2.66 <1.08 1610 - 73
TaC 13.90 6.46 3880 - 366(24°C)
14
Table 2.3 (cont’d)
TaSi2 9.1 10.80 2200 - 338(1260°C)
ThO2 9.86 9.54 3300 193(1090°C) 200(1090°C)
TiB2 4.50 8.28 2920 - 414(1090°C)
TiN 5.44 8.10 2950 1298 390
TiC 4.93 7.60 2830 55(1090°C) 296(24°C)
UO2 10.96 9.54 2847 - 172(1090°C)
VC 5.77 7.16 2699 - 434(24°C)
WC 15.63 5.09 2800 - 669(24°C)
WSi2 9.40 9.00 2165 - 248(1090°C)
ZrB2 6.09 8.28 1845 - 503(24°C)
ZrC 6.73 6.66 1852 90(1090°C) 359(24°C)
ZrN 7.35 7.0 2952 - 328
ZrO2 5.89 12.01 2700 83(1090°C) 132(1090°C)
Reinforcements used in the composites include carbides, nitrides, borides and
oxides. Among these reinforcements, titanium diboride (TiB2) can interact well
with the matrix, and it does not react with aluminum, which avoids the formation
of brittle interfacial products at the particle/matrix interface. Furthermore, the TiB2
particles exhibit very high stiffness and hardness. Given these favorable properties,
the TiB2 particles have received more attention than other reinforcements [23],
[24].
15
2.4 Physical and Mechanical Properties of Metal Matrix Composites
2.4.1 Physical Properties
Thermal Expansion Coefficient
“Reinforcement of light metal alloys with ceramic fibers or particles entails a
reduction in the thermal expansion coefficients. Simple models are available to
estimate the thermal expansion coefficients with the help of the characteristics of
the individual components” [25].
The simplest model is the rule of mixture (ROM) which is shown below in
Equation 2.1;
αc = αm Vm + αrVr (2.1)
where α is the thermal expansion coefficient, V is the volume fraction and the
subscripts c, m, r denote to the composite, matrix and reinforcing phase. The ROM
has some limitations and more complex models are available; Tuener [26] takes
into account the effects of isostatic stress in Equation 2.2;
CTE =
(2.2)
where K is the bulk modulus, V is the volume fraction and the subscripts c, m, r
denote the composite, matrix and reinforcing phase. This model gives more precise
results in the calculation of CTE than the ROM. Kener [27] offers the most
complex model that includes the effect of shear stress between matrix and
(isotropic and spherical) reinforcements which is shown below in Equation 2.3;
CTE = CTE − V (CTE − CTE ) × (2.3)
where;
A =K 3K + 4μ +(K − K )× 16μ + 12μ K
16
B = 3K + 4μ 4V μ (K − K ) + 3K K + 4μ K
“where µ and K are the shear and bulk modulus respectively and the subscripts m,
r denote matrix and reinforcing phase. The results of this model are between the
Tuener’s model and the ROM.” [28]
2.4.2 Mechanical Properties
2.4.2.1 Strengthening by Particles
“High modulus ceramic particles significantly increase hardness and strength of
the ductile aluminum matrix. One micromechanical model showing this effect is
described [29], [30] by Equation 2.4”:
∆R , =∆σα + ∆σ + ∆σ + ∆σ (2.4)
where ∆R , is the boost in tensile strength of aluminum matrix by the presence of
ceramic reinforcement; ∆σα etcetera are described below
The effect of induced dislocations, ∆σα ,is given by Equation 2.5:
∆σα = αGbp / (2.5)
with;
p = 12∆T ∆ (2.6)
“where ∆σα is the yield strength contribution due to geometrical necessary
dislocations and inner tension, α is a constant (values 0.5–1), G is the shear
modulus, b is the Burger’s vector, p is the dislocation density, ∆T is the
temperature difference, ∆C is the difference in thermal expansion coefficient
between matrix and particle, ΦP is the particle volume content and d the particle
size.”
The effect of the grain size, ∆σ , is given by Equation 2.7:
17
∆σ = k D / (2.7)
with
D = d ΦΦ
(2.8)
where ∆σ is the yield strength contribution from changes in grain size; kY1 is a
constant, D is the resulting grain size and Φ is the particle volume content.
The effect of the grain size ∆σ is given by Equation 2.9:
∆σ = k D / (2.9)
with
D = d (2.10)
“where ∆σ is the yield strength contribution due to changes in subgrain size
(for instance in a relaxation process during thermomechanical treatment of
composite), kY2 is a constant (typical value 0.05 MN m–3/2 ), Ds is the resulting
subgrain size and Φ is the particle volume content.”
The yield point is the yield strength at the 0.2 % elongation. The considerable
strain hardening is seen which is dependent on the particle diameter and content.
The strain hardening contribution ∆σ is given in equation 2.11;
∆σ = 퐾퐺Φ /ε / (2.11)
“where K is a constant, G the shear modulus, Φ the particle volume content, b the
Burger’s vector, d the particle diameter and Ɛ the elongation.”
“Generally higher hardening contributions are made by smaller particle diameters
than by coarser particles. For smaller particle diameters the work hardening and
18
the grain size influence contribute the most to the increase in the yield strength ”
[28].
2.4.2.2 Young’s Modulus (Modulus of Elasticity)
“A significant increase in the elastic modulus is observed with addition of
discontinuous reinforced ceramic particles. Reinforcement amount, geometry,
distribution and type affect the modulus. The suitable rules for the determination
of the modulus in the composite materials are given below” [31].
Inverse mixture rule: Reuss-model (IMR) is given by Equation 2.12
E = Φ + Φ (2.12)
“where Φ is the volume content of particles or fibers, Ec the Young’s modulus of
the composite material, EP the Young’s modulus of the particle or fiber and EM the
Young’s modulus of the matrix.”
“An advancement of these models, which is also applicable for short fibers or
particles, is the model by Tsai Halpin. By implementing an effective geometry
factor, which can be determined from the structure of the composite materials as a
function of the load direction, the geometry and the orientation of the
reinforcement can be considered” [32]:
E = ( Φ )Φ
(2.13)
with;
q = ( ⁄ )( ⁄ )
(2.14)
where S is the geometry factor of the fiber or particle (1/d).
“The critical requirement for the application of such models is the presence of a
composite material with an optimal structure, i.e. without pores, agglomerates of
particles or nonreinforced areas” [28].
19
2.5 Thermodynamics of Aluminum Matrix Composites
In spite of many processing techniques developed, mechanisms of the reactions
that occur in the process are not well understood. For example, in Al-Ti-B system,
several different reactions have been proposed between Al, Ti and B [33].
Maxwell and Hellawell [34] suggested a ternary peritectic reaction of
L (liquid) + Al3Ti + TiB2 = (Al) above 665°C and a ternary eutectic reaction of
L (liquid) = (Al) + Al3Ti + TiB2 below 659°C.
Abdel Hamid and Durand [35] proposed two transition reactions of
L + Al3Ti = (Al) + TiB2 and L + TiB2 = (Al) + AlB2 to take place within the
temperature range of 659±665°C. Thermodynamic calculations [36] have
consistently revealed the existence of compounds AlB2, TiB2, Al3Ti and mixed
boride phase of (Al, Ti)B2. Several studies on Al based metal matrix composites
(MMCs) reinforced by TiB2 were restricted to experimental investigations. A long
traditional debate has been made [37],[38] on whether AlB2 and TiB2 be present as
two separate phases or as a continuous series of solid solution of the form
(Al,Ti)B2. A number of investigators [39],[38] have reported that AlB2 and TiB2
can coexist together. Both AlB2 and TiB2 are of the same crystal structures and
they have lattice parameters that are very similar.
TiO2 and B2O3 are dissolved in cryolite based slag and overall reaction, between
the dissolved species; TiO2, B2O3 and liquid Al, is given below:
TiO2 (s) + B2O3 (l) + 10/3 Al (l) = TiB2 (s) + 5/3 Al2O3 (s)
with;
∆G° = - 1855133.3+331.3T [40],
Temperature range of the reaction is 933°K-1939°K (660⁰C -1680⁰C). ∆G° is
negative at all temperatures in the range. According to reaction which is shown
above, TiB2(s) particles can be produced. This reaction takes place either in single
step or multiple steps. TiO2 and B2O3 are reduced subsequent to the reaction given
above, Al2O3 remains at the system and will not involve in further chemical
20
reaction. In this system, only Al, Ti and B are available to take part in further
reactions. Ti and B are soluble in Al (l) . In the liquid aluminum bath, Ti and B can
react and such a reaction is given below:
[Ti]+2[B] =TiB2 (s) with ∆G = - 284,500 + 20.5T [41]
∆G of this intermediate reaction is negative at 1273°K (1000⁰C). According to
reaction is shown above, TiB2 (s) particles can be formed in the liquid aluminum.
2.6 Solubilities of Phases in Cryolite and Liquid Aluminum
2.6.1 Al2O3–Na3AlF6 and TiO2–Na3AlF6 binary systems
The solubility of alumina (=Al2O3) in the liquid cryolite is very important aspect
for the production of Aluminum via the famous Hall-Heroult process.
In Figure 2.4, the phase diagram of cryolite - alumina system is shown. Solubility
of Al2O3 can be determined for each temperature from the phase diagram [42].
Figure 2. 4 Alumina Solubility in Cryolite [42]
21
The solubility of TiO2 in Na3AlF6 can find out with phase diagram of
Cryolite – TiO2 The binary phase diagram is shown in Figure 2.5 [43].
Figure 2. 5 Cryolite-Titanium dioxide System [43]
2.6.2 TiB2―Al system
At 1300°K the solubility of TiB2 in molten Aluminum is 6X10-3 %wt. The
equilibrium is established within 10h between TiB2 and liquid Aluminum in a
library cell. At 1300°K, the aluminum solubility in TiB2 is trace amount so that it
is unmeasured [44] . Therefore, according to reaction given below produced
TiB2(s) particles are directly precipitated in liquid Aluminum.
[Ti]+2[B] =TiB2 (s) with ∆G = −284,500 + 20.5T [41]
[] denotes that species are dissolved in liquid aluminum.
22
2.6.3. Al―Ti, Al―B, Ti―B binary systems and Al―Ti―B ternary system
Titanium has some solubility in liquid aluminum at temperatures between 850°C
and 1200°C. Above 1200°C solubility of Ti increases rapidly but experimental
temperature range is limited at 1100°C.
In binary Al-Ti phase diagram, solubility limits and solid phases are shown in
Figure 2.6
Figure 2. 6 Al-Ti binary phase diagram [36].
23
Boron also has some solubility in liquid aluminum. From the Al-B phase diagram
given below, solubility of boron in liquid aluminum is seen to increase
significantly with increase in temperature.
Figure 2. 7 Al-B binary phase diagram [45].
24
In Ti-B binary phase diagram, which is given in Figure 2.8, TiB2(s) phase is stable
at between 30.1 and 31.1 weight % boron from 25°C to 2201°C. Ti3B4(s) and
TiB(s) phases will be formed with decreasing weight % boron. On the other hand,
elemental boron and TiB2(s) will be formed with increasing weight % boron.
Figure 2. 8 Ti-B binary phase diagram [45].
25
Al-Ti-B ternary phase diagram is given below in Figure 2.9:
Figure 2. 9 Al-B-Ti Ternary phase diagram [45].
2.7 Processing Methods for Particulate Reinforced Al Alloy Matrix MMCS
The early studies are concentrated on the fiber (continuously) reinforced aluminum
matrix composites however present studies are focused on particle or whisker
(discontinuously) reinforced aluminum matrix composites. They have
uncomplicated manufacture, low cost production, and isotropic mechanical
properties [46].
26
Classification of processing are made by synthesis of reinforcing phase for particle
reinforced MMCs. The processes can be divided two main classes;
External production of the reinforcement and its addition to matrix
(Ex-Situ),
Internal production of reinforcement (In-Situ).
In the ex-situ production, the reinforcing phase is produced separately out of the
matrix phase and it is introduced to the matrix with a secondary process. To the
contrary, in in-situ production, the reinforcing phase is produced directly within
the matrix.
Physical state of matrix is also accepted as a classification criterion. Three general
processing techniques are available according to physical state of matrix. These
are solid phase, liquid phase, and two phases. Classification is given in Figure 2.10
Figure 2. 10 Primary processing techniques of Al-MMCs according to physical state of matrix
27
2.7.1 Liquid State Processes
2.7.1.1 Casting
There are some basic different modifications between the conventional aluminum
casting and the high-quality composite casting. The differences in composite
foundry practices are:
The inert gas coverage can be applied because common degassing
procedures, such as gas injection or plunging tablets, can cause gas bubble
nucleation on the reinforcement particles and dewetting of the
reinforcement.
Temperature control is sensitive for the reinforcement phase, for example,
if overheating takes place, aluminum carbide will form in SiC reinforced
aluminum according to reaction; 4Al + 3SiC→Al4C3 + 3Si. Aluminum
carbide forms slightly above 750°C, and the reaction is fast in the 780-
800°C temperature range.
Stirring is crucial for the uniform distribution of ceramic particles. The
ceramic reinforcing particles neither melt nor dissolved in the liquid
aluminum matrix. Furthermore some ceramic particles are denser than the
liquid metal so the sinking of the particles can take place to the bottom of
the crucible.
Turbulence throughout the casting process can cause gas entrapment [46] .
Melting of Aluminum MMCs is identical to the conventional aluminum alloys.
Gas or oil fired, electric-resistance, and induction furnaces can be used. The
furnace is charged with dry composite ingots and then protective inert gas is
applied prior the melting. All equipments, such as ladles, skimmers,
thermocouples, also should be fully dried and preheated prior to use [46].
Generally density of ceramic reinforcements is higher than the aluminum alloys.
For instance the density of TiB2 is ~4.8 g/cm3 while density of aluminum alloys is
around 2.7 g/cm3. The recommendation is that the suitable stirring method is
applied for dispersing the reinforcements throughout the liquid metal. Speed of the
28
string is also important. Vortex formation can cause contamination from the dross
and entrapment of gas from the ambient atmosphere [46] .
2.7.1.2 Pressure Infiltration Casting
Pressure infiltration casting (PIC) is a unique MMC production technique. An
isostatical gas pressure, which is subjected to liquid metal, is used in order to fill
the particulate or fiber preform. Near-net shape production ,which can be made
with PIC, is crucial for decreasing machining requirement [46] .
Numerous methods are available with PIC for the discontinuously reinforced
MMCs. All of them include liquid metal infiltration into evacuated and free
standing preform by an external isostatically applied inert gas. The reinforcement
preform is placed into a mold, which is in the outer metal canister, prior to starting
of the PIC process. The metal canister serves to vacuum both inside of the mold
and preform during the applied pressure. Molten aluminum is placed at the entry
part of the mold and it is sealed. Schematic illustration of top-fill pressure
infiltration casting machine is shown below in Figure 2.11 [46].
Figure 2. 11 Schematic of structure of the top-fill PIC method
29
In the top-fill method, aluminum is melted in different crucible and liquid is
poured into the canister. After sufficient vacuum is maintained, the tube is sealed
and process started [46].
Generally the reinforcement amount changes from a minimum of 30% over than
70% because current technology limits the production of lower amounts. These
amounts serve stable geometrical shapes of preforms. SiC and Al2O3 are the most
common used reinforcement types for the PIC process [46].
2.7.1.3 In-Situ
In the in-situ production of composites, reinforcing phases (generally ceramic
materials) are produced directly within the aluminum matrix. To the contrary, ex-
situ composites, reinforcing phases are separately produced and at least one
secondary process is required to incorporation step [47]. There are numerous in
situ production techniques which are adopted from the conventional casting or
powder metallurgy [48].
The potential advantages of in situ production are uniform size and distribution of
reinforcements, clean interface between the reinforcement and matrix which
provides excellent interfacial bonding. Moreover, in this method, very fine
reinforcement sizes are achieved, about 0.5-5μm, which improves mechanical
properties of MMCs [47].
In ex-situ process, preserving and handling of micro-sized reinforcement are main
problems. Small sized particles (around ~ 1μm) are extremely reactive. This may
cause contamination of interface and unwanted brittle intermetallics can be
produced. Moreover, agglomeration of particles depresses mechanical properties
[49].
30
2.7.1.4 Wettability between Reinforcement and Matrix
“Wettability can be represented by an edge angle adjustment of a liquid droplet on
a solid substrate which is given by Young equation :
훾 −훾 = 훾 ∙ cos 휃 (2.18)
where θ is the edge angle, 훾 is the surface energy of the solid phase, 훾 is the
interface energy between the liquid and solid phases, and 훾 is the surface energy
of the liquid phase” [50].
Figure 2.12 illustrates the edge angle change of liquid droplet on the solid
substrate for various interfacial energy values. “ The wettable system is described
at an angle of <π/2. On the contrary, the nonwettable system is described at an
angle of >π/2. The critical angle value is π/2. The wettability increases with
decreasing the edge angle values” [50].
Figure 2. 12 Edge angle adjustment of a melt drop on a solid base for various values of the interface energy (after Young) [50].
31
In Table 2.7 surface and interface stresses of special metal-ceramic systems are
given at different temperatures.
Table 2.4 Surface and interface strains of selected metal–ceramic systems at different temperatures
Alloy, Ceramic, Systems
Temperature (°K)
γla (mJ m–2)
γsa (mJ m–2)
γls (mJ m–2)
Al 953 1050 –
Mg 943 560 – –
Al2O3 0 – 930 –
MgO 0 – 1150 –
Cu/Al2O3 1370 1308 1485 2541
Cu/Al2O3 1450 1292 1422 2284
Ni/Al2O3 1843 1751 1114 2204
Ni/Al2O3 2 03 176 9 1598
Al/SiC 973 851 2 69 2949
Al/SiC 1073 840 2414 2773
Al/SiC 1173 830 2350 2684
“As the contact develops, for example at the beginning of an infiltration, adhesion
occurs. The adhesion work WA for separation is” [51]:
W = γ −γ = γ (2.19)
W = γ ∙ (1 + cos θ) (2.20)
“In the case of immersion the interface between the solid and the atmosphere
disappears, while the interface between the solid and the liquid forms. The
immersing work WI is” [50]:
W = γ −γ (2.21)
32
“In the case of spreading the liquid is spread out on a solid surface. During this
procedure the solid surface is reduced as well as a new liquid surface being formed
and hence a new solid/liquid interface is formed. The spreading work WS is” [50]:
W = γ −γ − γ (2.22)
2.8 Solidification of Aluminum Matrix Composites and Distribution of
Ceramic Particles
Particulate reinforced hypoeutectic Al-Si matrix composites have important micro-
structural properties such as, eutectic Si morphology, primary α secondary dendrite
arm spacing, and distribution of reinforcing particles. Microstructures directly
affect the mechanical properties of composites. There are critical relationships
between solidification mechanisms and microstructural properties. Solidification
process of aluminum-TiB2 composites is more complex than the monolithic
aluminum alloys. Size, shape, and distribution of the reinforcement particles are
crucial for mechanical properties and also ceramic particles act as suitable
heterogeneous nucleation sites. Therefore, average grain size and secondary
dendrite arm spacing of primary hypoeutectic α can be reduced.
The distribution of reinforcement particles in the aluminum alloy matrix is directly
related with solidification. If the density difference between the liquid alloy and
the reinforcement particle is over 2 g/cm3, 2 μm diameter particles have a
10-4 cm/min sinking rate, therefore 1-2 μm diameter particles can be assumed as
suspending in the liquid metal [52]. In addition, the wetting phenomenon among
the reinforcement and liquid metal also cause preventing the sinking of reinforcing
particles within the liquid aluminum. If the reinforcing particles are distributed
uniformly in the liquid, agglomerations could be prevented.
In aluminum matrix composites growth of the solid is encountered under two
different reinforcement types. These are mobile and stationary reinforcements.
Generally fibers or monofilaments are stationary ones and whisker or particulates
are mobile reinforcements. In this study solidification takes place with the mobile
reinforcements. A growing solid-liquid boundary encounters mobile reinforcing
33
particles which are suspended in the liquid metal. The reinforcement can either be
engulfed or rejected. If the first case takes place, the distribution of reinforcement
in the newly formed solid will be uniform but slightly different than that in the
liquid. However in some cases, little redistribution may take place. On the other
hand, if reinforcements are pushed by the solid-liquid boundary, redistribution
occurs and final liquid regions will collect the whole reinforcing particles.
If the interface is planar, the reinforcing particles are not dragged and segregation
of particles in the latest solidification area is prevented. If cellular interface growth
takes place, particle segregations occur at the grain boundaries. In conclusion,
Interdendritic (nonuniform) segregation is a result of the nonplanar solid-liquid
interface and presence of convection in the liquid metal. reference [53].
Three conclusions can be made according to the experimental studies:
1. If the net Gibbs free energy change is negative (the particle liquid
interfacial energy is higher than the particle solid interfacial energy), the
particles are engulfed in all growth circumstances [54].
2. If the growth velocity is lower than the critical one, VC, particle is rejected.
On the contrary, at higher growth velocities than the VC, engulfing is
achieved. VC is dependent on the interface chemistry, thermal
conductivities of particles and matrix, temperature gradient, and liquid
viscosity [54].
3. Some reinforcing particles and matrix pairs usually reject and push the
particles at any growth situation. For instance, most of Al2O3, SiC, TiB2
and ZrB2- reinforced hypoeutectic Al alloys have nonplanar interfaces [54].
34
2.9 Interfaces in Aluminum Matrix Composites
In theory, interfaces are volumeless, two-dimensional regions where at least two
different materials meet and discontinuities occur in one or more material.
Nevertheless in reality there is always some volume existing within the interface
over which a gradual transition in material properties is present. Two major points
that emphasize the significance of interfaces in composites [55]:
Composites have significant interface area.
In ceramic reinforced MMC, excessive physical and mechanical property
differences are available between reinforcing particles and metallic matrixes.
Thermal expansion coefficient and elastic modulus are the main discontinuities of
the ceramic-metal system. During temperature changes resulting from
solidification or heat treatment, tension stress fields develop due to expansion of
metal matrix being larger than the ceramic reinforcement [55].
For an efficient load transfer between matrix and reinforcement, clean interface
must be established and preserved during the service conditions. Chemical or
physical bonds can be established. Chemical bonds are stronger but at perfect
wetting situations, physical bonds for instance Van Der Waals types might be
enough to produce a well bonded interface [56]. The interfacial reactions between
the reinforcement and the matrix deteriorate the interface quality. Unwanted brittle
intermetallic products such as Al3Ti and Al2B are source of the micro-structural
corruption. It is essential to understand the thermodynamic, kinetic, mechanical
and micro-structural aspects of interface in order to optimize the production to
achieve excellent interfaces in MMC’s [55].
35
CHAPTER III
EXPERIMENTAL PROCEDURE
3.1 Introduction
In this study, starting ingredients are TiO2, H3BO3, Na3AlF6 powders and
commercially pure Aluminum metal. They were charged into graphite crucible and
melted in an induction furnace.
The two-liquid phase system was maintained, in which liquid aluminum phase was
on the bottom and TiO2, H3BO3, and Na3AlF6 mixture slag phase was on the top.
TiB2 was formed through the overall slag-metal Reaction 3.1 as is shown below:
(B2O3) + (TiO2) + 10/3 Al (l) = TiB2(s) + 5/3 (Al2O3) (3.1)
where; the () denotes the species dissolved in the slag phase. After formation of
TiB2(s) particles, penetration of TiB2(s) to liquid metal takes place. (Al2O3)
dissolves in the slag phase and does not enter the liquid metal. After the reaction
was complete, the slag phase was removed carefully by a steel pot. Then the
composite, Al+TiB2, was poured and solidified into a graphite mold. Specimens
were carefully cut then X-Ray studies were carried out.
Afterwards coarse powder silicon and bulk magnesium additions were applied to
remelted Al+TiB2 in order to obtain A356, Al-(7 %wt.) Si-(0.3 %wt.) Mg alloy
matrix+TiB2 composite. Then, resulting A356 Aluminum alloy + TiB2 reinforced
composite was cast via squeeze casting method.
36
The squeeze cast samples were carefully prepared by suitable metallographical
techniques. And then, optical, scanning electron microscopic, and x-ray
examinations were carried out. Finally, mechanical tests such as Vickers
microhardness, tensile and, 3-point bending tests were done according to relevant
ASTM Standards.
3.2 Starting Materials
Commercial purity of aluminum ingots (> 99.7%wt.), rutile (TiO2, high purity >
99.8%wt.), boric acid (H3BO3, 1µm > particle size, high purity > 99.8%wt.), and
cryolite (Na3AlF6, 1µm > particle size, high purity > 99.8%wt.) were starting
ingredients for this study. Boric acid and rutile powders were obtained from the
Merck and the cryolite powder was obtained from Etibank. Information about the
powder products was obtained from the companies, and they indicated that the size
distributions for rutile and boric acid powders are similar and classified as fine.
3.3 Production Parameters
The starting materials, in powder form, TiO2, H3BO3, and Na3AlF6 and aluminum
ingots, were weighed according to the stoichiometry of reactions which are shown
below:
2H3BO3 = 3H2O + B2O3 (3.2)
TiO2 + B2O3 + 10/3 Al = TiB2 + 5/3 Al2O3 (3.3)
Three powder mixtures were prepared for the different, 5, 7.5, and 10 volume
percent TiB2 reinforcements. The raw powders, were carefully mechanically
mixed and then dried at 150°C for 4 hours in order to remove the humidity
thoroughly. Then the dried powder mixture and aluminum ingots were placed in
conductive graphite crucible. The nonconductive powders cannot be heated at
induction furnace (Figure 3.1) without conductive crucible. After the heating of
crucible had taken place by the secondary electric currents inside of the graphite,
the powders were heated and finally melted. Meanwhile, boric acid was calcined
37
to water vapor (H2O) and boric oxide (B2O3). Temperature was increased up to
1050°C-1100°C; the two-phase system was maintained: at the bottom, liquid
aluminum and at the top, liquid slag (TiO2, B2O3, Na3AlF6). The schematic
illustration of the slag-metal system is shown in Figure 3.1.
Figure 3. 1 The schematic illustration of the slag-metal system in the graphite crucible
Tiny sparks were observed when the exothermic metal slag reactions started. In
order to prevent excessive heating of the system, the furnace was turned down
when the reaction started. The system was carefully stirred for increasing the
contact between the slag and the metal. The stirring was continued until no sparks
were observed which was a sign of the completion of the reaction.
During the reaction, TiB2 particles were produced and then entered into the
aluminum melt. Aluminum oxide was formed and dissolved in the cryolite based
slag. The removal of aluminum oxide (Al2O3) was achieved by high alumina
solubility of cryolite based slag and in addition, the density difference of slag and
aluminum melt. For each three %vol. TiB2, three different reaction times were
applied, which were 15 min., 45 min, and 70 min . The temperature of the system
was fixed at 1100°C. After the reaction was complete, the slag phase was removed
carefully by a steel pot. Then the composite was poured and solidified in a graphite
mold.
Slag-Metal Rxn. @ 1100°C
TiO2 (s) + B2O3 (l) + 10/3 Al (l) = TiB2 (s) + 5/3 Al2O3 (s)
38
In order to examine the produced TiB2 reinforcing particles, the commercially pure
aluminum matrix+TiB2 composite was leached with a suitable acid solution, which
dissolved Al but did not affect TiB2. Hydrochloric acid (HCl) was used to
accomplish this. In order to extract TiB2 particles from the aluminum matrix,
leaching was carried out at 100°C, for 4 hours, and stirring was applied. After
dissolving the composite material in 6 molar HCl solutions, undissolved residue
was separated by filtration. Then TiB2 residue was washed, dried and then
weighted.
The residue was then subjected to XRD analysis. The XRD pattern of the residue
indicated that it did not contain any compound in any appreciable quantity other
than TiB2. The residue was considered to be pure TiB2 and its weight was found to
be close to the weight of TiB2 expected to form on the basis of stoichiometric
considerations. Moreover, after the pouring of commercially pure aluminum
matrix+TiB2 composite, the weighing was carefully applied in order to determine
TiB2 amount in the matrix.
Alloying procedure was carried out; silicon and magnesium were added to liquid
composite prior to the squeeze casting. Chemical compositions of matrix were
determined with FOUNDARY MASTER Model, optical emission spectrometric
device. The matrix alloy was well-known commercial aluminum casting alloy
A356 which consisted of the following major alloying elements; 7% wt. Si, 0.3%
wt. Mg.
3.4 Casting Practice and Squeeze Casting Machine
Prior to the casting, the temperature of the liquid A356 alloy- TiB2 composite was
kept at around 700-750°C. The hot worked tool steel die, which is designed for
two tensile specimens and four bending specimens, was pre-heated to 230-290°C.
39
The special die is shown in Figure 3.2.
Figure 3. 2 The hot worked tool steel die is designed for two tensile specimens and four bending specimens
In order to obtain fine microstructure and better mechanical properties, squeeze
casting of A356+TiB2 was applied under the 90 X 103 kgf for approximately 90
seconds to complete solidification before the composite was ejected. Squeeze
casting machine is shown below in Figure 3.3.
Figure 3. 3 Squeeze casting machine with hot worked steel die
40
Preheating of mold is crucial for the fluidity of liquid composite within the die. In
each casting practice, temperature of die was measured with special laser based
thermometer and temperature values of the die for prior to each casting is given
below in Table 3.1.
Table 3. 1 Pre-heating Die Temperatures
Reference Alloy
(A356)
A356+5%vol
TiB2
A356+7.55%vol
TiB2
A356+10%vol
TiB2
Casting
No Temperature °C Temperature °C Temperature °C Temperature °C
1 235 240 235 255
2 240 245 240 260
3 255 255 250 260
4 255 260 255 270
5 260 260 255 270
6 270 275 260 275
7 265 275 270 280
8 265 280 275 280
9 275 275 275 275
10 270 275 275 275
Finally the two tensile and four three point bending testing samples were taken out
from the steel die and cooled down in air.
41
3.5 Determination of Parent Phases in A356-TiB2 Composite via X-Ray
Diffraction Analysis
Bulk samples were carefully prepared with 120, 240, 320, 600, 800 and 1200 grid
SiC emery papers respectively. Then x-ray studies were conducted prior to
alloying and also after the alloying and squeeze casting. Therefore the phases were
observed at every step of the process. Moreover, if alloying elements such as
silicon and magnesium had formed any brittle intermetallic compounds with
aluminum or TiB2, they could be seen. RIGAKU X-Ray Diffraction equipment
with with Cu Kα radiation at 40 kV and 40 mA was used to attain the XRD
patterns.
3.6 Metallographic Examination
Metallographic samples were carefully cut from the edges of tension and bending
specimens by A BUHLER Metallographic cutter. Then samples were mounted in
bakalite in A METKON mounting machine. In order to apply metallographic
examination, samples were prepared using standard SiC grinding papers 120, 240,
320, 500, 600, 800, 1000, 1200, and 2000 grid respectively and polishing is
applied with 3 µm and 1 µm diamond suspension. Then etching was done with
Keller’ reagent (1 ml HF + 1.5 ml HCl + 2.5 ml HNO3 + 95 ml H2O ) in order to
observe grain structures and TiB2-matal matrix interfaces. The prepared samples
were investigated and photographed using NIKON ECLIPSE LV 150 special
optical metallographic microscope. Then scanning electron microscope studies
were carried out using JEOL 6400 scanning electron microscope and also energy
dispersive X-rays spectroscopy (EDS) analysis were taken from the TiB2
reinforcements with NORAN apparatus.
3.7 Secondary Dendrite Arm Spacing Distance (SDAS) Measurement
The common mechanical properties especially strength is directly related to
secondary dendrite arm spacing distance (SDAS) of the Al-Si alloy. The same
argument applies to the aluminum matrix composites. The strength increases with
42
decreasing SDAS. X-Rays and optical digital imaging software were used for
measurement of SDAS in the Al-Si+TiB2 composites.
Strict diffraction condition was explained by Bragg`s law, which is shown below
in Equation 3.4. In ideal case, infinitely thick crystal is one which contains no
imperfections and truly monochromic X-ray source. Peak broadening takes place
with increasing SDAS size. This occurs in such a manner that the width of peaks
measured at half height B can be related to cell or subgrain size, t, “SDAS” with
t = 0.9 λ / B cos θB 4(3.5)
where; λ is the wavelength of the X-ray, B is the structural broadening, and θB is
the Bragg’s angle of the peak.
In this study, a wide angle range scan was applied for the first sample and the
broadening peak was measured and selected. The (200) crystal plane peak was
selected and shown on Figure 3.4 below. The broadening could be measured at
half-height, but this is not enough as there were other factors that influence. One
factor was that the X-Ray source was not monochromatic as Kα in the doubled Kα1
and Kα2, other factors were geometric and depended on the alignment and slit
arrangement in the diffractometer. All these could be referred to as instrumental
broadening. Simple way of evaluating the degree of instrumental broadening was
to inspect a standard sample in the same setup. In the present experiment this was
achieved by examining an annealed aluminum (at 450oC for 20 hour) for this
purpose, which gave a SDAS size. Because SDAS were large, the broadening
obtained in this sample is due solely to an instrumental broadening. With such
standard sample, structural broadening, B, could be calculated in Equation 3.5 as
follows:
B = B −B (3.5)
Where; BM is the broadening of the sample, and BS is the broadening of the
standard.
43
Figure 3. 4 (200) crystal plane peak of Aluminum
3.8 Mechanical Tests
3.8.1 Tensile Test
Test specimens, which were prepared according to ASTM D3552 “testing
unidirectional or cross-piled metal matrix composites and discontinuous metal
matrix composites in tension”, was prepared during the squeeze/pressure die
casting. Tension test was applied for three different volume percentage of titanium
diboride (% TiB2 vol.). In Figure 3.5 the specimen geometry is shown clearly. The
test was carried out with SCHMATZU TENSILE TESTER with 3mm/min test
speed. In order to accurate elastic modulus calculations, the video was used during
the all tension tests.
Figure 3.5 The schematic of the tensile test specimen produced by squeeze/pressure die casting machine
90.5 mm
9 mm
30 mm
5.8 mm
5.8 mm
44
3.8.2 Three Point Bending Test
The test specimens, which were shown on Figure 3.6, were produced by
squeeze/pressure die casting machine.Test was carried out according to ASTM E
290 – 97a Standard Test Methods for Bend Testing of Material for Ductility. The
three point bending test is a well-known mechanical strength test especially
discontinuously reinforced metal matrix composites. Three point bending tests
were performed to observe the fracture behavior of aluminum matrix composite
with three different volume percentage of titanium diboride (% TiB2 vol.). The
three point bending test, the maximum fracture loads were evaluated. These
kilogram values were converted into flexural stress (MPa) values.
The flexural stress formula is given as in equation 3.6; σ = My/I were σ flexural
stress, M the bending moment, y the distance from the natural axis and I the
moment of inertia. The maximum flexural surface occurs in the mid-point of the
specimen.
Therefore:
M = × ; y = ; I = × ; σ = × ×× ×
(3.6)
P : Load applied by the testing machine,
t : Thickness of the specimen,
b : Width of the specimen, and
L : span length, respectively.
45
Figure 3. 6 The schematic of the three point bending test specimen specimens produced by squeeze/pressure die casting machine
3.8.3 Vickers Micro-Hardness Test
Vickers micro-hardness Test determined the hardness values of the metallic matrix
composites. Hardness measurements were carried out according to ASTM
standard E 92-82 (Reapproved 2003) describing standard test method for Vickers
hardness of metallic materials. Hardness Numbers (HV) were obtained with a
square pyramidal diamond indenter having included face angles of 136°, computed
from equation 3.7. Test was carried out at X200 magnification under 0.2 kgf load
for 15 seconds penetration time. Mounted, polished and etched specimens were
conducted test with standard Vickers indenter and Zwick/Roell ZHV 10
Microhardness machine which is shown in Figure 3.5.
HV = 2P sin α d = . (3.7)
where:
P= applied force, kgf,
d= mean diagonal of impression, mm, and
α= face angle of diamond = 136°.
50 mm , span length
10 mm
65 mm
5 mm
46
CHAPTER IV
RESULTS AND DISCUSSION
4.1 Formation of In-Situ TiB2 Reinforcing Particles and X-Ray Studies
Aim of the study was to produce in situ Al-TiB2 composites via method of the slag
metal reaction. Reactions were carried out with liquid aluminum, TiO2, and B2O3.
The cryolite (Na3AlF6) based slag contained TiO2 and B2O3. Moderately low
melting point and high Al2O3, TiO2, and B2O3 solubility are important parameters
for the cryolite based slag.
First of all, calcination of boric acid (H3BO3) took place during heating according
to reaction 4.1 which is shown below:
2H3BO3 (s) = 3H2O (g) + B2O3 (s) (4.1)
Second of all, melting of the slag phase took place at around 950°C and two liquid
phases which were liquid aluminum and liquid Na3AlF6-TiO2-B2O3 slag were
formed.
Finally, overall reaction started which was between the slag and metal phases for
the production of TiB2 as given by reaction 4.2. The charges were prepared for 5,
7.5, and 10 volume percent TiB2 reinforcement in aluminum matrix composites
according to the stoichiometry of the overall reaction 4.2.
TiO2 (s) + B2O3 (s) + 10/3 Al (l) = TiB2 (s)+ 5/3 Al2O3(s) (4.2)
47
The produced TiB2 particles entered the liquid aluminum and then became
reinforcement of the composite. The unwanted Al2O3 entered the slag phase and
dissolved in the cryolite according to Reaction 4.3.
2Al2O3 + 2Na3AlF6 = 4AlF3 + Na2Al2O4 + 2Na2O (4.3)
Weights of the cryolite (Na3AlF6), rutile (TiO2) and boric acid (H3BO3) powders
and aluminum metal charges are given in Table 4.1.
Table 4 1 Weight of the powders and aluminum metal charges
Vol %
TiB2
Na3AlF6
(gr)
TiO2
(gr)
H3BO3
(gr)
Al
(gr)
5 1931 101 156 1114
7.5 2974 155 240 1174
10 4075 213 330.0 1240
Before the alloying procedure, the commercially pure aluminum matrix–TiB2
composite was cast in a graphite mold. In order to identify produced phases in
every step of the process, x-ray studies were conducted both on the as-cast
composite and the alloyed squeeze cast composite. Then specimens obtained from
the squeeze cast A356 Aluminum alloy - TiB2 composite were subjected to optical
microscopy and scanning electron microscopy examinations and microhardness,
tensile and 3-point bending tests.
48
X-Ray diffraction pattern of commercially pure Aluminum, and commercially
pure Al+5 vol. % TiB2 samples are given below for three different reaction times
25 min., 45 min. , 70 min. in Figures 4.1 , 4.2, 4.3 and 4.4 ,respectively, while the
diffractometric data for TiB2 and Al given in Table 4.2 .
Table 4.2 Diffractometric Data for Pure Al-TiB2 As-Cast Composite
Al ● TiB2 ♦
2θ hkl 2θ hkl
38.324 111 27.469 001
44.563 200 33.949 100
64.948 220 44.652 101
78.069 311 56.748 002
82.263 222 60.999 110
67.969 102
68.357 111
71.342 200
78.672 201
88.189 112
49
Figure 4. 1 Al- 0% vol. TiB2 Commercially Pure Aluminum
Figure 4. 2 Al- 5% vol. TiB2 and 25 min. reaction time
0
5000
10000
15000
20000
25000
30000
20 30 40 50 60 70 80 902θ
Al(111)
Al(220)
Al(311)
Al(200)
Al(222)
0
5000
10000
15000
20000
25000
30000
35000
20 30 40 50 60 70 80 902θ
Al(111)
Al(220)
Al(311)
Al(200)
Al(222)TiB2
(001)TiB2(100)
TiB2(002)
TiB2(110)
TiB2
(102)TiB2(112)
TiB2(200)
TiB2(111)
Inte
nsity
In
tens
ity
50
Figure 4. 3 Al- 5% vol. TiB2 and 45 min. reaction time
Figure 4. 4 Al- 5% vol. TiB2 and 70 min. reaction time
0
5000
10000
15000
20000
25000
30000
35000
40000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222) TiB2
(001) TiB2
(111) TiB2
(002) TiB2
(100) TiB2
(110) TiB2
(200) TiB2
(112)
TiB2(201)
0
5000
10000
15000
20000
25000
30000
35000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222) TiB2
(001) TiB2
(102) TiB2
(200)
TiB2(100)
TiB2(110)
TiB2(111)
TiB2(112)
Inte
nsity
In
tens
ity
51
X-Ray diffraction pattern of Al-7.5 vol. % TiB2 samples are given for three
different reaction times 25 min., 45 min. , 70 min. in Figures 4.5 , 4.6, and 4.7
respectively.
Figure 4. 5 Al- 7.5% vol. TiB2 and 25 min. reaction time
Figure 4. 6 Al- 7.5% vol. TiB2 and 45 min. reaction time
0
5000
10000
15000
20000
25000
30000
35000
40000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222) TiB2
(001) TiB2
(102) TiB2
(200) TiB2
(100) TiB2
(110) TiB2
(112) TiB2
(002)
0
5000
10000
15000
20000
25000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222) TiB2
(001) TiB2
(002) TiB2
(200)
TiB2(100)
TiB2(111)
TiB2(112)
TiB2(110)
Inte
nsity
Inte
nsity
52
Figure 4. 7 Al- 7.5% vol. TiB2 and 70 min. reaction time
X-Ray diffraction pattern of Al-10 vol. % TiB2 samples are given for three
different reaction times 25 min., 45 min. and, 70 min. respectively in Figures 4.8 ,
4.9, and 4.10 respectively.
Figure 4. 8 Al- 10% vol. TiB2 and 25 min. reaction time
0
5000
10000
15000
20000
25000
30000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222) TiB2
(001) TiB2
(002) TiB2
(200)
TiB2(100) TiB2
(111) TiB2
(112) TiB2
(110)
0
5000
10000
15000
20000
25000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222)
TiB2(001)
TiB2(002)
TiB2(200)
TiB2(100) TiB2
(111) TiB2
(112) TiB2
(110)
Inte
nsity
Inte
nsity
53
Figure 4. 9 Al- 10% vol. TiB2 and 45 min. reaction time
Figure 4. 10 Al- 10% vol. TiB2 and 70 min. reaction time
X-Ray diffraction patterns given in Figures 4.2 to 4.10 clearly indicate that TiB2 is
present in the aluminum specimens, which clearly indicate that Al-TiB2
composites have been successfully produced. Intermetallic Ti-Al, especially Al3Ti,
0
5000
10000
15000
20000
25000
30000
35000
40000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222)
TiB2(001)
TiB2(002)
TiB2(200)
TiB2(100) TiB2
(111) TiB2
(112) TiB2
(110)
0
2000
4000
6000
8000
10000
12000
20 30 40 50 60 70 80 902θ
Al (111) Al
(200) Al (220)
Al (311)
Al (222)
TiB2(001)
TiB2(002)
TiB2(200)
TiB2(100)
TiB2(111)
TiB2(112)
TiB2(110)
Inte
nsity
Inte
nsity
54
phases which may form in the system and have been observed to form in some
studies, Al2O3 and any other compound could not be detected in the commercially
pure aluminum-TiB2 samples. This is proved by the XRD patterns which are
shown in Figures 4.2 to 4.20. XRD patterns show that none or a negligible amount
of Ti-Al intermetallic compound has formed and all Al2O3 forming has entered the
liquid cryolite phase.
X-Ray diffraction patterns of A356 (Al7Si0.3Mg) -5% vol.TiB2 , A356-7.5%
vol.TiB2 and A356-10% vol.TiB2 squeeze cast samples are given in Figures 4.12,
4.13, and 4.14 respectively, while the diffractometric data for TiB2 and A356
given in Table 4.3 .
Also the diffraction pattern of A356 alloy is given in Figure 4.11 in order to
compare of the TiB2 peaks in the alloy.
Table 4.3 Diffractometric Data for A356-TiB2 Squeeze Cast Composite
TiB2 ♦ Al ● Si ■
2θ hkl 2θ hkl 2θ hkl
27.324 001 38.501 111 28.442 111
33.981 100 44.541 200 47.302 220
56.783 002 65.082 220 56.121 311
61.041 110 78.221 311 69.132 400
68.142 102 82.041 222 76.377 331
71.581 200 80.026 422
88.321 112
55
Figure 4. 11 A356 -0% vol.TiB2
Figure 4. 12 A356 -5% vol.TiB2
0
5000
10000
15000
20000
25000
30000
20 30 40 50 60 70 80 902θ
Si (111)
Al (111)
Al (200)
Al (311)
Al (220) Al
(222) Si (220)
Si (311)
Si (400)
Si (422)
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222) TiB2
(001) TiB2(002)
TiB2(100)
TiB2(102) TiB2
(112) TiB2
(110) Si
(111)
Si (220) Si
(311) Si
(400)
Si (422)
Inte
nsity
Inte
nsity
56
Figure 4. 13 A356 -7.5% vol.TiB2
Figure 4. 14 A356 -10% vol.TiB2
0
2000
4000
6000
8000
10000
12000
14000
16000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222)
TiB2(001)
TiB2(002) TiB2
(100) TiB2
(112) TiB2(102)
TiB2(110)
Si (111)
Si (220)
Si (311)
Si (400)
Si (311)
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20 30 40 50 60 70 80 902θ
Al (111)
Al (200)
Al (220)
Al (311)
Al (222)
TiB2(001)
TiB2(002)
TiB2(100) TiB2
(102) TiB2
(112) TiB2
(110)
Si (111) Si
(220)
Si (311) Si
(400)
Inte
nsity
Inte
nsity
57
X-Ray diffraction patterns given in Figures 4.12 to 4.14 obviously point out that
TiB2 is present in the aluminum specimens, which clearly indicate that A356-TiB2
squeeze cast composites have been successfully produced. Aluminum, Silicon, and
TiB2 peaks appear in the diffractogram, but magnesium peaks cannot be seen
because of its low amount in the alloy system.
The alloying elements, Si and Mg do not form any intermetallic phases with TiB2.
According to the X-Ray patterns which are shown on Figures 4.12 to 4.14, the
only appreciable phase is TiB2 and any other ceramic or intermetallic phases are
not present in the final metallic matrix composite system.
4.2 Determination the Amount of TiB2 Reinforcement
In the present study, three different charges were prepared on the basis of
stoichiometric considerations based on Reaction 4.2. X-Ray patterns indicate the
presence of TiB2; unfortunately it does not give information as to the amount of
TiB2 in the samples.
In order to determine the amount of TiB2 in the matrix extraction of TiB2 particles
was applied. The method was leaching the bulk composite with a suitable agent,
which would dissolve Al but would not affect TiB2 reinforcement. Hydrochloric
acid (HCl) was selected to fulfill this objective. After dissolving the bulk
composite in HCl, undissolved residue was separated from the solution by
filtering. The residue was washed with distilled water, dried at 120°C and then
weighed. The residue was then subjected to XRD analysis. The XRD pattern of the
residue and its diffractometric data are shown in Figure 4.15 and in Table 4.4.
XRD analysis indicated that composite did not contain any compound in any
appreciable quantity other than TiB2.
58
Figure 4. 15 XRD pattern of leach residue of TiB2 powder
Table 4.4 Diffractometric Data of TiB2 Leach Residue
TiB2 ♦
2θ 27.62 34.18 44.48 57.02 61.16 68.14
hkl 001 100 101 002 110 102
2θ 68.14 68.32 71.92 78.66 88.38
hkl 102 111 200 201 112
The residue was considered to be pure TiB2 and its weight was taken as the weight
of TiB2 that has formed.
Efficiency of formation of TiB2 was defined as:
퐸푓푓푖푐푖푒푛푐푦 =experimentalweightoftheproducedamountofTiB
stoichiometricamountofTiB (accordingtoreaction4.2)푥100
0
5000
10000
15000
20000
25000
20 30 40 50 60 70 80 902θ
TiB2(001)
TiB2(111)
TiB2(002)
TiB2(100)
TiB2(110)
TiB2(200)
TiB2(112)
TiB2(201)
TiB2(101)
TiB2(102)
Inte
nsity
59
The X-ray studies of the slag phase clearly showed that slag does not have any
significant amount of TiB2. Figure 4.16 shows XRD pattern of slag phase. No TiB2
peak is observed the XRD pattern of slag phase.
Figure 4. 16 XRD pattern of slag phase containing Al2O3 and Na3AlF6
Efficiency results are shown on Table 4.5 for each experiment. Efficiency of the
TiB2 production is increased with increasing reaction times which is an expected
result.
Table 4.5 The Efficiencies of the TiB2 production via Slag-Metal reaction
Experiment
No
Time
(min)
Temperature
(°C)
%vol. TiB2 % Efficiency
1 25 1100 5 83
2 45 1100 5 92
3 70 1100 5 97
4 25 1100 7.5 78
5 45 1100 7.5 89
02000400060008000
1000012000140001600018000
20 30 40 50 60 70 80 90
2θ
Inte
nsity
60
Table 4.5 (cont’d)
6 70 1100 7.5 96
7 25 1100 10 80
8 45 1100 10 84
9 70 1100 10 92
4.3 Microstructures and Phase Relations
In this study TiB2 is used as a discontinuous ceramic reinforcement and also an
additive for grain refinement. All of the samples were ground using SiC grinding
paper of 120, 220, 320, 600, 800 and 1200 grid and they were then polished with 3
micron and 1 micron oil based diamond suspension and then the surfaces were
cleaned in alcohol with ultrasonic cleaner. Afterwards specimens were etched with
Keller’s reagent.
Optical emission spectroscopic analysis was used to determine chemical
composition of the prepared aluminum silicon alloys. Results are given below in
Table 4.6:
Table 4.6. Chemical Compositions of Prepared Metal Matrix Composite
Samples
Element (Wt.%)
A356 Ref. Sample
A356-Vol. 5%TiB2
A356-Vol. 7.5%TiB2
A356-Vol. 10%TiB2
Si 7.32 7.84 8.48 8.84
Mg 0.299 0.315 0.296 0.302
Fe 0.196 0.256 0.370 0.360
Cu 0.004 0.013 0.010 0.013
61
Table 4.6 (cont’d)
Mn 0.001 0.001 0.001 0.001
Zn 0.005 0.005 0.011 0.198
Cr 0.005 0.017 0.018 0.017
Ni 0.005 0.018 0.055 0.043
Al Balance
Optical and scanning electron microscopy studies were conducted on squeeze cast
A356 alloy containing no TiB2 and squeeze cast A356-TiB2 composite samples
containing different amounts of TiB2. Micrographs are given in Figure 4.17 to
4.20.
The squeeze cast microstructure of A356 alloy containing no TiB2, which is shown
on Figure 4.17, consists of a eutectic phase located in between the fine primary α-
aluminum dendrites.
a X100 b X200
Figure 4. 17 Optical micrographs of A356 alloy with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000, Samples were etched with Keller’s reagent
Proeutectic α dendrites
62
c X500 d X1000
igure 4. 17 Optical micrographs of A356 alloy with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000, Samples were etched with Keller’s reagent (continued)
The optical photomicrographs of A356-5 vol. % TiB2 squeeze cast composite is
given in Figure 4.18 with four different magnifications.
a X100 b X200
Figure 4. 18 Optical micrographs of A356-5vol. % TiB2 with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000, Samples were etched with Keller’s reagent
Proeutectic α dendrites
Eutectic Si & TiB2 particles
Proeutectic α dendrites
Eutectic Si needles
63
c X500 d X1000
Figure 4. 18 Optical micrographs of A356-5vol. % TiB2 with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000, Samples were etched with Keller’s reagent (continued)
The optical photomicrographs of A356-7.5 vol. % TiB2 squeeze cast composite is
given in Figure 4.19 with four different magnifications.
a X100 b X200
Figure 4. 19 Optical micrographs of A356-7.5 vol. % TiB2 with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000 , Samples were etched with Keller’s reagent
Proeutectic α dendrites
Proeutectic α dendrites
Eutectic Si & TiB2 particles
Eutectic Si & TiB2 particles
Proeutectic α dendrites
64
c X500 d X1000
Figure 4. 19 Optical micrographs of A356-7.5 vol. % TiB2 with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000 , Samples were etched with Keller’s reagent (continued)
The optical photomicrographs of A356-10 vol. % TiB2 squeeze cast composite is
given in Figure 4.20 with four different magnifications.
a X100 b X200
Figure 4. 20 Optical micrographs of A356-10 vol. % TiB2 with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000 , Samples were etched with Keller’s reagent
Proeutectic α dendrites
Eutectic Si & TiB2 particles
Proeutectic α dendrites
Eutectic Si & TiB2 particles
65
c X500 d X1000
Figure 4. 20 Optical micrographs of A356-10 vol. % TiB2 with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000 , Samples were etched with Keller’s reagent (continued)
In order to observe coarse eutectic silicon around the large proeutectic α aluminum
dendrites in plain A356 alloy. Annealing procedure was applied at 450°C for 20
hours. The main aim of the annealing was removal of the squeeze effect on the
eutectic silicon. Sand-cast A356 alloy consists of coarse eutectic silicon needles
around large proeutectic α-aluminum dendrites. Such a microstructure was not
observed in the squeeze-cast samples of this study.
The optical photomicrograph of A356, which was annealed at 450°C for 20 hours,
is given in Figure 4.21 with four different magnifications.
a X100 b X200
Figure 4. 21 Optical micrographs of annealed (at 450°C for 20 hours),A356 reference specimen with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000 , Samples were etched with Keller’s reagent
Proeutectic α dendrites
Eutectic Si needles
Proeutectic α dendrites
Eutectic Si & TiB2 particles
66
c X500 d X1000
Figure 4. 21 Optical micrographs of annealed (at 450°C for 20 hours),A356 reference specimen with different magnifications respectively; (a) X100, (b) X200, (c) X500, and (d) X1000 , Samples were etched with Keller’s reagent (continued)
In Figure 4.21 the eutectic silicon needles are seen to be 30µm -70µm long; clearly
seen especially at X500 and X1000 magnifications. The major differences between
squeeze cast A356 alloy (Figure 4.17) and annealed A356 alloy (Figure 4.21) are
the morphology of the eutectic silicon and the proeutectic α-Al dendrite size. In
squeeze cast sample, the eutectic Si morphology was observed in moderately small
rods around the fine proeutectic α dendrites. On the other hand, in annealed sample
the long needle eutectic morphology is seen. In addition the proeutectic
Al-dendrite size also decreased with the squeeze casting method.
In Figure 4.17, eutectic Si was observed as non modified small flakes in the
squeeze cast A356 alloy. The modification action, which is shown in Figures 4.18
to 4.20, was performed by TiB2 particles during solidification. Fine fibrous
eutectic Si was obtained in eutectic Si modified composite samples.
Distribution of TiB2 reinforcements is concentrated on the interdendritic regions
(between the first solidified α-dendrites); the size of the reinforcement particles is
smaller than 3-4 µm. The pro-eutectic α dendrites does not contain TiB2 particles
according to EDS point analysis results.
Coarse eutectic Si needles length: 30 -70µm Proeutectic
α dendrites
67
The optical micrographs suggest that while amount of the TiB2 particles increases,
the interdendritic regions become concentrated with TiB2 particles. Moreover, size
of the Si particles decreases progressively with TiB2 addition within the
interdendritic region. The reason should be; above the eutectic temperature, 577°C,
the liquid contains first solidified parts which are proeutectic α-dendrites and TiB2
reinforcing particles. Fine ceramic particles restrict the growth of α-dendrites.
Furthermore, below the 577°C liquid contains α-dendrites, fine TiB2 particles and
eutectic Si. Again, ceramic particles restrict eutectic Si growth within the
interdendritic regions.
A comparison of A356- vol. 5%TiB2 (Fig. 4.18 c and d) and A356-vol. 10% TiB2
composites (Fig. 4.20 c and d) clearly suggest that with the increasing number of
TiB2 particles, the primary α dendrite size and dendrite arm spacing decreases
progressively. The continuous network structure comprising of “TiB2 and Si
particles” becomes more prominent in A356- vol. 10% TiB2 composite which is
shown clearly in Figure 4.20b, 4.20c and 4.20d.
68
The scanning electron microscopy studies are given in Figure 4.22 to 4.25. The
photomicrographs clearly indicated that TiB2 particles and fibrous eutectic Si were
located around the primary α dendrites.
a X3000
b X4000 Figure 4. 22 A356 0% vol TiB2 Squeeze cast, Samples were etched with Keller’s reagent
1
2
Eutectic Si-Phase
α-Al Dendrites
Eutectic Si Needles
69
a X5300
b X11800
Figure 4. 23 A356-5% vol TiB2 , Squeeze cast Samples were etched with Keller’s reagent
TiB2 Particles
α-Al Dendrites
Eutectic Fibrous Si Particles
Eutectic Fibrous Si Particles
α-Al Dendrites
1
3
2
70
a X3000
b X5000
Figure 4. 24 A356-7.5% vol TiB2 , Squeeze cast, Samples were etched with Keller’s reagent
Eutectic phase mixture &TiB2
Primary α-aluminum grains
Primary α-aluminum grains
1
2
Eutectic Si phase &TiB2 Particles
71
a X3000
b X5000
Figure 4. 25 A356-10% vol TiB2 , Squeeze cast Samples were etched with Keller’s reagent
Eutectic phase mixture &TiB2
Primary α-aluminum grains
2
1
Eutectic Si phase &TiB2
Primary α-aluminum grains
72
Primary α dendrites are clearly seen as dark regions on SEM micrographs. The
dendrite size and arm spacing decreases progressively with increasing TiB2
amount which is shown on Figure 4.22a, Figure 4.23a, Figure 4.24a and Figure
4.25a. In SEM micrographs Figure 4.24b and 4.25b with higher magnifications,
TiB2 reinforcements are clearly shown as white particles around the dendrites.
Generally the size of the particles are less than 4 µm and around the 1 µm. Around
the pro-eutectic α dendrites, TiB2 distribution is mostly uniform and agglomeration
of particles did not take place during the solidification, on the other hand TiB2
particles cannot penetrate into the proeutectic Al dendrites. This is shown on
Figures 4.23a, 4.24b and 4.25b.
In Figure 4.22a, Points 1 and 2 shows that EDS analyses were taken in SEM
micrograph of A356 alloy. According to EDS Figures 4.26a and 4.26b, the black
dendritic areas consist of Aluminum proeutectic dendrite was marked point 1 and
between the black areas the eutectic silicon phase was marked point 2.
TiB2 particles are observed in the SEM micrographs given in Figure 4.23a, 4.24 b
and 4.25 b. EDS analysis, which are given in Figure 4.27, Figure 4.28 and
Figure 4.29. They were indicated that points 1, 2 and 3 are TiB2 particles on the
three micrograph, (Figure 4.23a, Figure 4.24b, and Figure 4.25b). On the basis of
XRD results of leach residue and EDS results of composite, these particles are
titanium diboride reinforcements.
According to EDS analysis given in Figures 4.27b, 4.28b and 4.29b, eutectic
fibrous Si and TiB2 particles are in the form of fine particle mixture which are
placed between the proeutectic α dendrites. The EDS analysis were taken on point
2 and 3, which are marked at Figures 4.23a, 4.24b and 4.25b
73
Element
Weight Conc %
Atom Conc %
Al 100.00 100.00
4.26 A A356 Primary Aluminum Dendrites at point 1
Element
Weight Conc %
Atom Conc %
Al 60.17 61.13
Si 39.83 38.87
4.26 B A356 Al-Si eutectic phase at point 2 on Figure 4.22a
Figure 4. 26 EDS point analyses on Figure 4.22a
74
Element
Weight Conc %
Atom Conc %
Al 99.48 99.71
Ti 0.52 0.29
4.27 A %5vol TiB2/A356 Primary Al-dendrites on point 1 at Figure 4.23a
Element
Weight
Conc %
Atom
Conc %
Al 73.29 77.53
Si 15.60 15.86
Ti 11.10 6.61
4.27 B %5vol TiB2/A356 Al-Si Eurectic Phase on point 2 at Figure 4.23a
Figure 4.27 EDS point analyses on vol.5% TiB2-A356 Alloy at Figure 4.23a
75
Element
Weight
Conc %
Atom
Conc %
Al 89.33 91.87
Si 4.78 4.72
Ti 5.89 3.41
4.27 C %5vol TiB2/A356 TiB2 particles on point 3 at Figure 4.23a
Figure 4. 27 EDS point analyses on vol.5% TiB2-A356 Alloy at Figure 4.23a (continued)
Element
Weight Conc
%
Atom Conc
%
B 12.27 36.21
Al 8.91 10.53
Si 1.58 1.80
Ti 77.24 51.45
4.28 A %7.5vol TiB2/A356 TiB2 particles on point 1 at Figure 4.24b
Figure 4.28 EDS point analyses on vol. 7.5% TiB2-A356 Alloy at Figure 4.24b
76
Element
Weight Conc %
Atom Conc %
B 10.23 30.64
Al 12.98 15.57
Si 3.95 4.56
Ti 72.84 49.23
Figure 4. 28 EDS point analyses on vol. 7.5% TiB2-A356 Alloy at Figure 4.24b (continued)
Element
Weight
Conc %
Atom
Conc %
B 65.80 73.02
Al 13.15 14.02
Si 18.74 11.71
Ti 2.31 1.24
4.28 B %7.5vol TiB2/A356 TiB2 particles on point 2 at Figure 4.24b
4.29 A %10 vol TiB2/A356 TiB2 particles on point 1at Figure 4.25b Figure 4.29 EDS point analyses on vol. 10% TiB2-A356 Alloy at Figure 4.25b
77
Element
Weight Conc %
Atom Conc %
B 10.23 30.74
Al 13.45 16.20
Si 2.74 3.16
Ti 73.58 49.90
Figure 4. 29 EDS point analyses on vol. 10% TiB2-A356 Alloy at Figure 4.25b (continued)
4.4 Secondary Dendrite Arm Spacing (SDAS) Measurement
In chapter 3.7 the main principles of the grain size determination were explained in
detail. Yield and ultimate tensile strengths and ductility increases with decrease in
secondary dendrite arm spacing in metal matrixes.
In order to determine secondary dendrite arm spacings, the broadening of the X-
ray diffraction peaks was measured. First of all, a reference sample which
contained no reinforcement was used.
The X-Ray pattern maximum intensity (200) peak of reference sample is given in
Figure 4.30 (a). Then, this sample was metallographically prepared by grinding,
polishing and micro etching with 100ml H2O + 200 ml HNO3 + 180 ml HCl + 60
gr FeCl3. In order to calculate the size of the grains, X50 micrograph of reference
sample was used. Lines with 1 cm intervals were drawn on the micrograph and the
number of grains that cut the lines was counted. The secondary dendrite arm
spacing of this sample was measured as 23.66 µm.
4.29 B %10 vol TiB2/A356 TiB2 particles on point 2 at Figure 4.25b
78
In Figure 4.30 (b), 4.30 (c) and 4.30 (d) the X-Ray pattern of 5 %, 7.5 % and 10 %
vol. TiB2, are presented. To establish the variation in the grain size of aluminum
alloy matrix by addition of 5 %, 7.5 % and 10 % vol. TiB2, the broadening of the
(200) peaks were measured.
a %0TiB2-A356 Reference Sample
b %5 vol.TiB2-A356
Figure 4. 30 Diffraction patterns of A356-TiB2 composite showing (200) peaks broadening because of secondary dendrite arm spacing’s
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
44 44,5 45 45,5 462θ
0
2000
4000
6000
8000
10000
12000
44 44,5 45 45,5 462θ
Inte
nsity
B=3.90X10-3 Rad.
At 6000hallf intensity
2θ Peak anlgle 44.8°
Inte
nsity
B
B=3.56X10-3 Rad.
At 8000hallf intensity
2θ Peak anlgle 44.75°
B
79
c %7.5 vol.TiB2-A356
d %10 vol.TiB2-A356
Figure 4. 30 Diffraction patterns of A356-TiB2 composite showing (200) peaks broadening because of secondary dendrite arm spacing’s (continued)
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
44 44,5 45 45,5 462θ
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
44 44,5 45 45,5 462θ
B
Inte
nsity
B=4.19X10-3 Rad.
At 4000hallf intensity
2θ Peak anlgle 44.77°
B=5.45X10-3 Rad.
At 2375 hallf intensity
2θ Peak anlgle 44.82°
B
Inte
nsity
80
Clearly seen in Figure 4.30, the broadening of the (200) peak increases with the
increase of the percentage of reinforcement TiB2 particles. To determine the grain
size of the aluminum alloy matrix for each sample, Equations 3.1 and 3.2 are given
in Chapter III were applied. Calculation results showed which are presented in
Table 4.7; the increase in the volume percentage of TiB2 causes a decrease in the
grain size.
In addition the broadening calculations, using the special metallographic
microscope and suitable software, the secondary dendrite arm spacing
measurement was performed and results are shown in Figure 4.31.
Figure 4. 31 Secondary Dendrite arm spacing measurement of A356 reference sample via to special metallographic light microscope and software. X200 magnification and etched with Keller’ reagent
Secondary Dendrite Arm Spacing Measurement
81
Secondary dendrite arm spacing results are given in Table 4.7.
Table 4.7 Reinforcement Content and SDAS Size*
Vol. % Reinforcement SDAS Size (µm)
0 23±3
5
[% Increment]
21±2
[-8.6]
7.5
[% Increment]
19±3
[-17.4]
10
[% Increment]
14±2
[-38.6]
*The broadening measurements were correlated to the experimental composite’s SDAS size
measured by optical microscope. Since the effect of subgrain reflections cannot be distinguished,
the SDAS size information deduced from the linear trend line is may not be the actual SDAS size.
4.5 Mechanical Properties of A356 –TiB2 Composites
The mechanical test results, which were conducted on the composite samples, are
summarized in Table 4.8. The values listed in the table indicate the average of 15
values determined for each parameter. The results indicate that the ultimate tensile
strength (UTS), yield strength (YS), modulus of elasticity (E, Young’s Modulus),
flexural strength and microhardness increase with increasing amount of TiB2
particles in the composite.
Table 4.8 Mechanical Testing Results of A356-TiB2 Composite
TiB2 reinforcement vol. %
0 5 7.5 10
UTS (MPa) [Increment %]
173 182±3 [+5]
192±2 [+9]
205±4 [+19]
82
Table 4.8 (cont’d) YS (MPa)
[Increment %] 148±2 159±2 [+7]
169±2 [+14]
174±3 [+18]
E (GPa) [Increment %] 19±2 22±1
[+15.7] 23±2
[+21.1] 25±1
[+31.1]
% Reduction in Area [Increment %] 10.2±2 9.7±2
[-4.9] 8.3±2 [-18.6]
7.5±1 [-26.5]
% Elongation [Increment %] 11.1±1 10.5±1
[-5.4] 9.2±1 [-17.1]
8.6±1 [-22.5]
Hardness (HV 0.2) [Increment %] 101±2 104±3
[+2.9] 111±3 [+9.9]
127±5 [+25.7]
Flexural Strength (MPa) [Increment %] 332±4 358±3
[+7.8] 372±3 [+12.0]
387±5 [+16.6]
The macro and micro hardness results are shown in Table 4.9. Micro-hardness test
was applied at X200 magnification. 200 grf test load was applied with common
Vickers indenter. Brinell macro-hardness test was applied with 500 kgf test load
and 10 mm steel ball indenter.
Table 4. 9 Brinell Macro and Vickers Micro Hardness Numbers of A356-TiB2 Composite
TiB2 reinforcement vol. %
HV 0.2 [Increment %]
HBS [Increment %]
0 101±2 88±2
5 104±3 [+2.9]
90±4 [+2.2]
7.5 111±3 [+9.9]
95±3 [+7.9]
10 127±5 [+25.7]
109±4 [+23.86]
83
Graphical forms of the results are shown in Figure 4.32. These graphs also indicate
that the ultimate tensile strength (UTS), yield strength (YS) and hardness (HV0.2)
increases with the increasing amount of particles in the composite.
(a)
Figure 4. 32 (a) Variation of ultimate tensile strength (UTS) with volume percent of reinforcements.(b)Variation of yield strength (YS) with volume percent of reinforcements.(c)Variation of flexural strength with volume percent of reinforcements.(d)Variation of hardness (HV0.2) with volume percent of reinforcements. (e) Variation of elastic modulus (E) with volume percent of reinforcements
-30
20
70
120
170
220
0 2,5 5 7,5 10
low stregth values dueto Hydrogen voidsU
TS
(MPa
)
vol. %TiB2
84
(b)
(c)
Figure 4. 32 (a) Variation of ultimate tensile strength (UTS) with volume percent of reinforcements.(b)Variation of yield strength (YS) with volume percent of reinforcements.(c)Variation of flexural strength with volume percent of reinforcements.(d)Variation of hardness (HV0.2) with volume percent of reinforcements. (e) Variation of elastic modulus (E) with volume percent of reinforcements (continued)
0
20
40
60
80
100
120
140
160
180
0 2,5 5 7,5 10
low stregth values due to Hydrogen gas voids
320
330
340
350
360
370
380
390
400
0 2,5 5 7,5 10
FS
(MPa
)
vol. %TiB2
vol. %TiB2
YS
(M
Pa)
85
(d) .
(e)
Figure 4. 32 (a) Variation of ultimate tensile strength (UTS) with volume percent of reinforcements.(b)Variation of yield strength (YS) with volume percent of reinforcements.(c)Variation of flexural strength with volume percent of reinforcements.(d)Variation of hardness (HV0.2) with volume percent of reinforcements. (e) Variation of elastic modulus (E) with volume percent of reinforcements (continued)
0
20
40
60
80
100
120
140
0 2,5 5 7,5 10
0
5
10
15
20
25
30
35
0 2,5 5 7,5 10
vol. %TiB2
HV
0.2
vol. %TiB2
E (G
Pa)
86
Some ultimate and yield strength values were abnormally low. These low data
were not included in average ultimate tensile and yield strength calculations.
Macro examinations were carried out on the fracture surface of these specimens.
Some voids were observed which are given below in Figure 4.33.
Figure 4. 33 Fracture surfaces of the defected tensile specimens
Most probably, Hydrogen picking of liquid aluminum caused these defects,
Because, hydrogen is the only gas which is soluble in liquid aluminum. With
increasing temperature hydrogen solubility increases enormously; this is shown
below in Figure 4.34.
Figure 4. 34 Hydrogen solubility (pH2=1 atm) with increasing temperature [57]
Hydrogen gas voids at the fracture surface
87
The ductility change is shown in Figure 4.35 in terms of reduction in area % and
elongation %, with amount of reinforcement. As it is seen, the ductility decreases
with the increase of reinforcement in composite.
(a)
(b)
Figure 4. 35. (a)Variation of elongation with volume percent of reinforcements. (b)Variation of ductility with volume percent of reinforcements
0
2
4
6
8
10
12
14
0 2,5 5 7,5 10
0,0
2,0
4,0
6,0
8,0
10,0
12,0
0 2,5 5 7,5 10
vol. %TiB2
vol. %TiB2
% E
long
atio
n %
R. A
rea
88
The results shown in Figure 4.32 and 4.35 were consistent with the strengthening
mechanisms of discontinuously reinforced metal matrix composites mentioned in
section 2.4.2.1 which is explained in detail. Four main aspects, which are directly
related with the mechanical properties and discontinuously reinforcing ceramic
particles, available for supporting mechanical results of the present study are:
Thermal expansion coefficients of ceramic TiB2 particles and A356 metal
matrix are 8.28 µm/m°K and 23.5 µm/m°K, respectively. During cooling,
metal matrix is more contracted than the discontinuously reinforced TiB2
particles. As a result of CTE difference, the plastic deformation zones and
high dislocation densities are created around the TiB2 particles. The
uniformly distributed TiB2 particles cause effective strengthening due to
the CTE mismatch.
During solidification, TiB2 particles generate heterogeneous nucleation
sites and produce considerably smaller grain size compared with the
unreinforced matrix alloy. The smaller grain size directly increases the
strength: this phenomenon is explained by the Hall-Petch formulation
which is initially developed for describing the strengthening due to the
pinning of the dislocations at the grain boundaries.
Eutectic silicon morphology is another crucial aspect for the strengthening
of Al-Si casting alloys. This is transformation of coarse silicon needles to
small fibrous ones. In the present study, TiB2 particles fulfilled this
modification. The difference is clearly seen in SEM micrographs; In Figure
4.22b coarse Si needles were observed on the specimen containing no TiB2
Modified Si particles were observed on specimen containing TiB2 as can be
seen in Figures 4.23, 4.24, 4.25.
Load transfer between matrix and the ceramic particles is other crucial
mechanism that is responsible for strengthening in the discontinuously
reinforced composites. Ductile A356 matrix and hard reinforcement is
excellent combination during the service loads.
89
Lastly, mechanical properties of A356-discontinuosy reinforced TiB2 composites
produced in present study were compared with the properties of composites
produced by other methods found in literature. Comparisons are summarized in
Table 4.10. Processing methods, such as heat treatment and extrusion, were
applied to the composites in these methods. As compared with the other methods,
the in-situ slag metal reaction method used in the current work supplies a more
significant increase in the strength parameters with the incremental increase in the
percentage of reinforcements.
Table 4.10 Comparison of Mechanical Properties of Al-TiB2 Metal Matrix Composites
Process Vol % Reinfor..
%EL %RA UTS MPa
YS MPa
Flexural Stress
Hardness HB
In-situ A356 - TiB2 Composite formed by slag-metal reaction (present work)
0 11.1 10.2 147 121 332 56
5 10.5 9.7 153 129 358 58
7.5 9.2 8.3 162 137 372 62
10 8.6 7.5 178 144 387 71 Al - 2%Mg - Al2O3 - TiB2 Composite
formed by melt string [58]
0 13.4 24.7 150 - - 39
3.54 10.3 16.7 164 - - 53
5.84 11.6 20.7 154 - - 44.5
Al - 3%Mg - Al2O3 - TiB2 Composite
formed by melt -stirring [22]
0 29.6 22.9 162 80.5 - 52.6
2.03 28.2 21.9 163 94.5 - 57.7
3.58 23.5 19.1 169 99.5 - 64.5
4.42 20 16.7 175 108 - 73.2
In-situ Al-4.5 Cu -TiB2 by in melt reaction
[59]
5 1.9 - 358 258 - 83
7 2.8 - 387 283 - 84
10 3.3 - 417 317 - 87 AA1100-TiB2
in-situ stir casting [60]
7.3 4.6 - 223 171.3 - -
Al - 4.5Cu-TiB2 in-situ stir
casting [60]
15 2.3 - 333 202.8 - -
Al- TiB2 Composite by stir casting[61]
5 9.2 - 124 96 - -
10 6.3 - 164 128 - -
15 5.5 - 153 124 - -
90
CHAPTER V
CONCLUSIONS
In the present study, discontinuously TiB2 reinforced aluminum A356 alloy matrix
composites were successfully produced and metallographic and mechanical
properties were carefully investigated. Slag/metal reaction method was applied
during the in-situ production route. Commercially pure Aluminum, Boric acid
(H3BO3), Rutile (TiO2) and Cryolite (Na3AlF6) were used as ingredient materials.
The amounts of TiB2 particles in the produced composites were 5%, 7.5%, 10% by
volume. The matrix was well known A 356 aluminum casting alloy which has 7
weight % Si and 0.3 weight % Mg. Squeeze casting process was applied and
mechanical testing specimens were prepared by use of a special die.
Microstructural studies were performed by both light and scanning electron
microscopes. In order to identify the phases present, and the microstructural
charteristics, energy dispersive X-ray spectroscopy and X-ray diffraction analysis
were conducted.
Tensile tests, three point bending tests and hardness tests were carefully carried
out. Furthermore, the effect of reinforcement amount on the mechanical properties
was determined.
91
The main results which were obtained in the present study are shown below:
1. TiB2 particles were formed in-situ in the aluminum matrix through the
reaction of TiO2 and B2O3 in the Na3AlF6 based slag by liquid Aluminum.
2. TiB2 particles formed by slag-metal reaction were found to have formed in
roughly spherical shapes having 1-4 μm in size.
3. TiB2 particles were found no to have penetrated into the first solidified
proeutectic α-dendrites. TiB2 particles were distributed around the proeutectic α-
dendrites and a more homogenous distribution compared with other processing
techniques such as solid state reactive sintering, was obtained.
4. Grains and dendrite arm spacings were significantly refined with the
formation of TiB2 particles.
5. The eutectic silicon phase was modified and the flake eutectic silicon
transformed into a fibrous form between the proeutectic α-Al dendrites.
6. Ultimate tensile strength, yield strength, flexural strength and hardness of
the material increased with increasing TiB2 reinforcement amount in the metallic
matrix.
7. Elongation and reduction in area decreased with increasing reinforcement
amount in the metallic matrix.
Finally, it can be stated that in-situ technique used in this study can successfully be
applied to produce an A356-TiB2 composite
92
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